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Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Absolute dating provides a numerical age or range, in contrast with relative dating, which places events in order without any measure of the age between events. Some scientists prefer the terms chronometric dating or calendar dating, as the use of the word "absolute" may imply an unwarranted certainty of accuracy.
In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (such as coins and historical records). For example, coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year. Absolute dating techniques include radiocarbon dating of wood or bones, potassium-argon dating, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics.
In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from younger organic remains (radiocarbon dating with 14C) to systems such as uranium–lead dating that allow determination of absolute ages for some of the oldest rocks on Earth.
== Radiometric techniques ==
Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes. Particular isotopes are suitable for different applications due to the types of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half-lives in the thousands of years, such as carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.
=== Radiocarbon dating ===
One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay. Cosmic radiation entering Earth's atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the uptake of carbon-14 stops.
It takes 5,730 years for half the carbon-14 to decay to nitrogen; this is the half-life of carbon-14. After another 5,730 years, only one-quarter of the original carbon-14 will remain. After yet another 5,730 years, only one-eighth will be left.
By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.
==== Limitations ====
The relatively short half-life of carbon-14, 5,730 years, makes dating reliable only up to about 60,000 years. The technique often cannot pinpoint the date of an archeological site better than historic records, but is highly effective for precise dates when calibrated with other dating techniques such as tree-ring dating.
An additional problem with carbon-14 dates from archeological sites is known as the "old wood" problem. In dry, desert climates, organic materials like dead trees can remain in their natural state for hundreds of years. When people eventually use these materials as firewood or building supplies, they become part of the archaeological record. Thus, dating that particular tree does not necessarily indicate when the fire burned or the structure was built.
For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.
=== Potassium-argon dating ===
Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium–argon dating (K–Ar dating). Potassium-40 is a radioactive isotope of potassium that decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated.
Argon, a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K–Ar dating was used to calibrate the geomagnetic polarity time scale.
== Luminescence dating ==
=== Thermoluminescence ===
Thermoluminescence dating also dates items to the last time they were heated. This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item.
Heating an item to 500 degrees Celsius or higher releases the trapped electrons, producing light. This light can be measured to determine the last time the item was heated.
Radiation levels do not remain constant over time. Fluctuating levels can skew results – for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.
Because of these and other factors, Thermoluminescence is at most about 15% accurate. It cannot be used to accurately date a site on its own. However, it can be used to confirm the antiquity of an item.

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=== Optically stimulated luminescence (OSL) ===
Optically stimulated luminescence (OSL) dating constrains the time at which sediment was last exposed to light. During sediment transport, exposure to sunlight 'zeros' the luminescence signal. Upon burial, the sediment accumulates a luminescence signal as natural ambient radiation gradually ionises the mineral grains.
Careful sampling under dark conditions allows the sediment to be exposed to artificial light in the laboratory, which releases the OSL signal. The amount of luminescence released is used to calculate the equivalent dose (De) that the sediment has acquired since deposition, which can be used in combination with the dose rate (Dr) to calculate the age.
== Dendrochronology ==
Dendrochronology, or tree-ring dating, is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year.
Dendrochronology has three main areas of application: paleoecology, where it is used to determine certain aspects of past ecologies (most prominently climate); archaeology, where it is used to date old buildings, etc.; and radiocarbon dating, where it is used to calibrate radiocarbon ages (see below).
In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands. Currently, the maximum for fully anchored chronologies is a little over 11,000 years from present.
== Amino acid dating ==
Amino acid dating is a dating technique used to estimate the age of a specimen in paleobiology, archaeology, forensic science, taphonomy, sedimentary geology and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed. All biological tissues contain amino acids. All amino acids except glycine (the simplest one) are optically active, having an asymmetric carbon atom. This means that the amino acid can have two different configurations, "D" or "L" which are mirror images of each other.
With a few important exceptions, living organisms keep all their amino acids in the "L" configuration. When an organism dies, control over the configuration of the amino acids ceases, and the ratio of D to L moves from a value near 0 towards an equilibrium value near 1, a process called racemization. Thus, measuring the ratio of D to L in a sample enables one to estimate how long ago the specimen died.
== See also ==
Astronomical chronology
Age of the Earth
Age of the universe
Chronological dating, archaeological chronology
Absolute dating, this article
Relative dating
Phase (archaeology)
Archaeological association
Geochronology
Chronostratigraphy
Future of the Earth
Geologic time scale
Geological history of Earth
Plate reconstruction
Plate tectonics
Thermochronology
Timeline of natural history
List of geochronologic names
General
Consilience, evidence from independent, unrelated sources can "converge" on strong conclusions
== References ==
== Further reading ==

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The Acali expedition (or Acali experiment or the Sex Raft) was a 1973 social experiment that aimed to investigate interpersonal relationships in conditions of limited space and social isolation. The experiment was conceived by Mexican anthropologist Santiago Genovés, who had previously been a crew member of Thor Heyerdahl's Ra expedition. The participants showed a restraint towards aggression, which frustrated Genovés and led him to start to try to create conflict, and at one point he took command of the float. Despite these attempts, the group remained peaceful.
The raft had a complement of eleven people: five men and six women. It left Las Palmas, Spain, on 12 May 1973 and took 101 days to drift across the Atlantic Ocean and reach Cozumel, Mexico, with a single stopover in Barbados. Frequently dubbed the "Sex Raft" by the media, it was the subject of a 2018 documentary film The Raft, by Marcus Lindeen.
== The Raft ==
The name of the raft, Acali, comes from the Nahuatl language and means "the house on the water".
The raft was built specifically for the experiment. It had a steel hull and dimensions of 12 by 7 metres. The cabin measured 4 × 4 metres. It was designed by José Antonio Mandri and Colin Mudie, and it was built in Newcastle upon Tyne, England.
== Participants ==
== See also ==
Stanford prison experiment (1971)
== References ==
== External links ==
Raft of Passion - Episode of Snap Judgement in which Mary Gidley recounts her experience with the Acali experiment
The Raft - Documentary film

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Acanthochronology is the study of cactus spines or Euphorbia thorns grown in time ordered sequence (i.e. in series). Physical, morphological or chemical characteristics and information about the relative order or absolute age of the spines or thorns is used to study past climate or plant physiology.
For example, columnar cactus spines grow from the apex of the plant. After several weeks the spines stop growing and have been moved to the side of the stem. The old spines remain in place for decades as new spines are created at the continually growing apex. The result is that along each external "rib" of the cactus is a series of spines arranged in the order they grew in – the oldest spines are at the bottom and the youngest spines are at the top. These spines can be dated using bomb-spike Carbon-14 and isotopes of carbon (Carbon-13) and oxygen (Oxygen-18) may be used to infer past climate (e.g. precipitation or temperature), plant stem growth or plant physiology (e.g. photosynthetic processes). Alternatively, the width of small transverse bands in the spine may be used to infer daily information about cloud cover or plant productivity, although this remains to be tested. It has also been shown that regular waxy banding on the sides of a Costa Rican cactus (Lemaireocereus aragonii) indicate annual growth and can be used as temporal chronometers.
This sub-discipline of paleoclimatology and ecophysiology is relatively new. Acanthochronology is closely related to dendrochronology, dendroclimatology and isotope geochemistry and borrows many of the methods and techniques from these sub-disciplines of the Earth Sciences. It also draws heavily from the field of ecophysiology, a branch of Biology, to ascribe spine or thorn characteristics to particular environmental or physiological variables.
The first peer-reviewed article to present and explain an isotope spine series was from a saguaro cactus in Tucson, Arizona. This and other work shows that radiocarbon and isotope time-series derived from spines can be used for demographic or palaeoclimate studies.
== References ==
== Further reading ==
Doménech-Carbó, Antonio (2015). "Dating: An analytical task". Chemtexts. 1. doi:10.1007/s40828-014-0005-6.

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The Aircraft Nuclear Propulsion (ANP) program and the preceding Nuclear Energy for the Propulsion of Aircraft (NEPA) project worked to develop a nuclear propulsion system for aircraft. The United States Army Air Forces initiated Project NEPA on May 28, 1946. NEPA operated until May 1951, when the project was transferred to the joint Atomic Energy Commission (AEC)/USAF ANP. The USAF pursued two different systems for nuclear-powered jet engines, the Direct Air Cycle concept, which was developed by General Electric, and Indirect Air Cycle, which was assigned to Pratt & Whitney. The program was intended to develop and test the Convair X-6, but was canceled in 1961 before that aircraft was built. The total cost of the program from 1946 to 1961 was about $1 billion.
== Types ==
=== Direct air cycle ===
Direct cycle nuclear engines resemble a conventional jet engine without combustion chambers. The hot compressed air produced by the compressor section is instead directed into the nuclear reactor core. The air is heated further, thereby cooling the reactor. The air is then expanded through a turbine, powering the compressor, before it is exhausted at high velocity to provide thrust. The end result is that instead of using jet fuel, an aircraft could rely on the heat from nuclear reactions.
The General Electric program, which was based at Evendale, Ohio, was pursued because of its advantages in simplicity, reliability, suitability and quick start ability. Conventional jet engine compressor and turbine sections were used, with the compressed air run through the reactor to be heated by it before being exhausted through the turbine.
=== Indirect air cycle ===
Indirect cycling involves thermal exchange outside of the core with compressor air being sent to a heat exchanger. The nuclear reactor core would heat up pressurized water or liquid metal and send it to the heat exchanger as well. That hot liquid would be cooled by the air; the air would be heated by the liquid, sent through a turbine (powering the compressor), then out the exhaust, providing thrust.
The indirect air cycle program was assigned to Pratt & Whitney, at a facility near Middletown, Connecticut. This concept would have produced far less radioactive pollution. One or two loops of liquid metal would carry the heat from the reactor to the engine. This program involved a great deal of research and development of many light-weight systems suitable for use in aircraft, such as heat exchangers, liquid-metal turbopumps and radiators. The Indirect Cycle program never came anywhere near producing flight-ready hardware.
== Experimental reactors and projects ==
=== Aircraft Reactor Experiment ===
The United States Aircraft Reactor Experiment (ARE) was a 2.5 MWth thermal-spectrum nuclear reactor experiment designed to attain a high power density and high output temperature for use as an engine in a nuclear-powered bomber aircraft. The advantage of a nuclear-powered aircraft over a conventionally-powered aircraft is that it could remain airborne orders of magnitude longer and provide an effective nuclear strategic deterrent to a nuclear-armed Soviet adversary. The ARE was the first molten salt reactor (MSR) to be built and operated. It used the molten fluoride salt NaF–ZrF4–UF4 (53–41–6 mol%) as fuel, was moderated by a hexagonal-configuration beryllium oxide (BeO), and had a peak temperature of 860 °C. A redundant liquid sodium coolant system was used to cool the moderator and reflector materials. A secondary helium gas coolant loop was circulated around the primary coolant to transfer heat to a water radiator where heat output was dumped to atmosphere. Reactivity control rods were installed and it was found that the control rods did not determine the output power of the ARE; rather, the power demand did, which affected the outlet and inlet temperatures because of the negative temperature coefficient of reactivity. The ARE was operated at power for 221 hours up to a peak of 2.5 MWth.
=== MX-1589 project ===
On September 5, 1951, the USAF awarded Convair a contract to fly a nuclear reactor on board a modified Convair B-36 Peacemaker under the MX-1589 project of the ANP program. The NB-36H Nuclear Test Aircraft (NTA) was to study shielding requirements for an airborne reactor, to determine whether a nuclear aircraft was feasible. This was the only known airborne reactor experiment by the U.S. with an operational nuclear reactor on board. The NTA flew a total of 47 times testing the reactor over West Texas and Southern New Mexico. The reactor, named the Aircraft Shield Test Reactor (ASTR), was operational but did not power the aircraft; the primary purpose of the flight program was testing the effectiveness of the shielding. Based on the results of the NTA, the X-6 and the entire nuclear aircraft program was abandoned in 1961.
=== Heat Transfer Reactor Experiments ===
As part of the AEC/USAF ANP program, in 1956 modified General Electric J47s were first operated on nuclear power using a reactor test assembly known as Heat Transfer Reactor Experiment 1 (HTRE-1). HTRE-1, which used vertically-oriented control rods, was reconfigured with a removable core to become HTRE-2 for additional testing. HTRE-3 was built separately to test horizontally-oriented control rods as appropriate for use in an airframe.
The decommissioned HTRE-2 and HTRE-3 reactors and test assemblies can be viewed by the public in the Experimental Breeder Reactor I parking lot at Idaho National Laboratory.

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=== Pratt & Whitney Aircraft Reactor-1 ===
On February 5, 1957, another reactor was made critical at the Critical Experiments Facility of the Oak Ridge National Laboratory (ORNL) as part of the circulating-fuel reactor program of the Pratt & Whitney Aircraft Company (PWAC). This was called the PWAR-1, the Pratt & Whitney Aircraft Reactor-1. The purpose of the experiment was to experimentally verify the theoretically predicted nuclear properties of a PWAC reactor. The experiment was only run briefly; by the end of February 1957 all data had been taken and disassembly had begun. The experiment was run at essentially zero nuclear power. The operating temperature was held constant at approximately 675 °C (1,247 °F), which corresponds closely to the design operating temperature of the PWAR-l moderator; this temperature was maintained by external heaters. Like the 2.5 MWt ARE, the PWAR-1 used NaF-ZrF4-UF4 as the primary fuel and coolant.
== Cancellation ==
Technological competition with the Soviet Union (as represented by the launch of Sputnik 1), and continued strong support from the Air Force allowed the program to continue, despite divided leadership between the DOD and the AEC. Numerous test facilities were funded and constructed through the 1950s and 1960–61 in order to produce a flight-worthy nuclear power unit, including one at the Oak Ridge National Laboratory (ORNL). While the ARE successfully demonstrated operation of a MSR concept, the program was canceled by President Kennedy on March 26, 1961, citing the high cost with no flight-worthy reactor having been produced up to that point – "15 years and about $1 billion have been devoted to the attempted development of a nuclear-powered aircraft; but the possibility of achieving a militarily useful aircraft in the foreseeable future is still very remote". Also contributing to the cancellation was that the first intercontinental ballistic missiles entered into active service in September 1959 which all but eliminated the need for a nuclear-powered aircraft as a strategic deterrent. Nevertheless, the results of the ARE program prompted scientists and engineers at ORNL to submit a preliminary design proposal to the Atomic Energy Commission for a 30 MWth experimental MSR to explore MSR as a civilian power station concept. The result of the proposal was direction from the Atomic Energy Commission for ORNL to design, construct, and operate the Molten-Salt Reactor Experiment (MSRE).
== See also ==
List of nuclear-powered aircraft
Georgia Nuclear Aircraft Laboratory
WS-125, 1955 USAF requirement for nuclear powered bomber
NERVA
Project Pluto to develop nuclear powered ramjet engines for use in cruise missiles
Project Rover to develop a nuclear thermal rocket
Tupolev Tu-95LAL
== References ==
== External links ==
Gantz, Kenneth (1960), Nuclear flight; the United States Air Force programs for atomic jets, missiles, and rockets., New York, Duell, Sloan and Pearce.
Thorton, G (June 28, 1962), Comprehensive Technical Report, General Electric Direct-Air-Cycle Aircraft Nuclear Propulsion Program, Program Summary and References, US Atomic Energy Commission (AEC), OSTI 1048124.
Dreams of Nuclear Flight — The NEPA and ANP programs (PDF), Wisc, archived from the original (PDF) on June 18, 2010, retrieved August 12, 2009.
The Bureau of Atomic Tourism, archived, archived from the original on December 31, 2010.
Wendt, Gerald (1951), A Scientist Preview The First Atomic Airplane (article) with illustrations on the subject of using an atomic reactor to power an aircraft.
Martin, Richard (May 8, 2012), "ANP", SuperFuel, St. Martin's Publishing, pp. 109–12, ISBN 9780230341913.
SOVİET TOP SECRET NUCLEAR AIRPLANE M-60 Akademi Portal by Akademi Portal web site (in English)
COMPREHENSHIVE TECHNICAL REPORT GE DIRECT AIR CYCLE AIRCRAFT NUCLEAR PROPULSION PROGRAM (in English)
"Flyable" Reactors & Neutron Coupling (in English)
Aircraft Nuclear Propulsion Program: Hearing before the Subcommittee on Research and Development of the Joint Committee on Atomic Energy (Report). July 23, 1959. hdl:2027/uiug.30112065524198.
Declassified Aircraft Nuclear Propulsion Program: Manned Aircraft Progress Report 1956-1958

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Amino acid dating or racemization dating is a dating technique used to estimate the age of a specimen in paleobiology, molecular paleontology, archaeology, forensic science, taphonomy, sedimentary geology and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed.
== Background ==
=== Chemistry ===
Amino acids are a set of organic compounds that are used by living organisms to synthesise proteins. All amino acids (except glycine) have one or more pairs of stereoisomers, isomers which share the same bond order but are organized differently in 3D space. Amino acid stereoisomer pairs that are optically active and non-superimposable mirror images of each other are enantiomers; pairs that are not mirror images are diastereomers or epimers. Biological systems are stereoselective, preferring certain stereoisomers for chemical reactions; living organisms keep all their amino acids in their "left-handed" (L or levo-) forms (a state called homochirality) because they are unable to use the "right-handed" (D or dextro-) forms for protein synthesis. This ratio of D and L forms is unstable, as the molecules may undergo reactions (known as racemization or epimerization respective to the type of stereoisomer pair involved) and become the other stereoisomer.
When an organism becomes unable to keep its amino acids in that unbalanced ratio, such as by dying or shedding tissue, the system will proceed towards chemical equilibrium. Measuring the progress of this interconversion reaction
allows estimation of an organism's time of death, if environmental variables like moisture and temperature are accounted for.
=== Amino acids and environmental conditions ===
Amino acids commonly used for amino acid dating analysis are leucine, aspartic acid, valine, glutamic acid, and diastereomer isoleucine.
The properties of the amino acid(s) chosen for analysis influence what kind of dating can be performed. Amino acid interconversion reactions happen at a variety of speeds: aspartic acid racemizes very quickly and hence is used for recent samples where high resolution is important, while valine and leucine take much longer to racemize and are more appropriate for older fossils. Additionally, these reaction rates are sensitive to temperature, to a degree depending on the specific interconversion reaction. The racemization rate of aspartic acid varies with small changes in temperature, while valine's racemization rate is less temperature dependent.
Besides higher temperatures accelerating interconversion, other environmental variables also impact reaction rates. Wetter environments produce faster reaction rates, and interconversion reactions may be catalyzed by the presence of acids, bases, or metal cations. The chosen host organisms or taxa also introduce bias into age estimates.
Amino acids which are bound within peptides interconvert more slowly than those which are free or are occupying the terminal position of peptide chains. The degree of hydrolysis of peptides (and therefore the speed at which equilibrium approaches) increases with fossil age.
== Applications ==
Amino acid dating has applications in archaeology, stratigraphy, oceanography, paleogeography, paleobiology, and paleoclimatology. These include dating correlation, relative dating, sedimentation rate analysis, sediment transport studies, conservation paleobiology, taphonomy and time-averaging,sea level determinations, and thermal history reconstructions.
Amino acid dating may be used to date samples too old for radiocarbon dating (which has a maximum range of 40 ka to 0 ka), or too young for potassium-argon dating (which has a range of 40 ka to 150 ka) to be helpful. Verification of radiocarbon and other dating techniques by comparison with amino acid dating is also possible. The 'filling in' of large probability ranges, such as those caused by variation in 14C levels throughout the biosphere, has sometimes been possible as well.
Bone, shell, and sediment studies have contributed much to the paleontological record, including that relating to hominoids. Many studies have been undertaken in paleopathology and dietary selection, paleozoogeography and indigeneity, taxonomy and taphonomy, and DNA viability. Human cultural changes and their effects on local ecologies have been assessed using this technique; the differentiation of cooked from uncooked bone, shell, and residue is sometimes possible.
Amino acid racemization also has a role in tissue and protein degradation studies, particularly useful in developing museum preservation methods. These studies have produced models of protein adhesive and other biopolymer deteriorations and the concurrent pore system development. The reduction in bodily repair capability during aging is important to studies of senescence and age-associated disease, and allows the determination of age in living animals.
Forensic science can use this technique to estimate the age of a cadaver or an objet d'art to determine authenticity.
== Methods ==
Amino acid racemization analysis consists of sample preparation, isolation of the amino acid wanted, and measure of its D:L ratio. Sample preparation entails the identification, raw extraction, and separation of proteins into their constituent amino acids, typically by grinding followed by acid hydrolysis. The amino acid derivative hydrolysis product can be combined with a chiral specific fluorescent, separated by chromatography or electrophoresis, and the particular amino acid D:L ratio determined by fluorescence. Alternatively, the particular amino acid can be separated by chromatography or electrophoresis, combined with a metal cation, and the D:L ratio determined by mass spectrometry.
Conventional racemization analysis tends to report a D-alloisoleucine / L-isoleucine (A/I or D/L ratio). This amino acid ratio has the advantages of being relatively easy to measure and being chronologically useful through the Quaternary.
Reversed phase HPLC techniques can measure up to 9 amino acids useful in geochronology over different time scales on a single chromatogram (aspartic acid, glutamic acid, serine, alanine, arginine, tyrosine, valine, phenylalanine, leucine).
Amino acid dating relies on the assumption that the fraction of amino acids being studied has been a closed system since its formation, exchanging nothing with its surroundings. Removing contaminants decreases variability in results by ensuring that analysis is performed only on the most representative fraction of amino acids. These cleaning methods may include soaking powdered biomineral samples in bleach prior to measuring D/L ratio, destroying the amino acids in the more porous, open areas while leaving the fraction trapped inside the grains unscathed.
== References ==
== External links ==
=== Active laboratories ===
Northern Arizona University Amino Acid Geochronology Laboratory Archived 2017-03-31 at the Wayback Machine
University of Massachusetts Amino Acid Geochronology Laboratory
The University of Colorado Amino Acid Geochronology Lab
University of Delaware Research Group
University of York BioArCh
Madrid School of Mines Biomolecular Stratigraphy Laboratory

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Appearance event ordination or AEO is a scientific method for biochronology through the ordering of the appearance of fossil mammal genera by multivariate analysis, using conjunctional (overlapping) and disconjunctional (nonoverlapping) range distributions in large sets of data.
== Process ==
AEO is based on faunal overlap and stratigraphic superposition to derive a best-fit sequence of first and last appearance events.
=== Step 1 ===
The first step is to translate patterns of overlap and superposition into pairwise first-before-last statements. The wolf species Canis edwardii and Canis armbrusteri are used as example taxa for the following patterns. Each statement means C. edwardii, for example, must have first appeared before C. armbusteri last appeared. This is true whenever either (1) C. edwardii and C. armbrusteri have been found together in at least one nontime-averaged fossil collection, or (2) C. edwardii is found lower in at least one lithostratigraphic section than C. armbrusteri.
=== Step 2 ===
A multivariate ordination algorithm is applied to derive a first-pass, hypothesized sequence of first and last appearances. The minimal constraint on this sequence is that if there is an observed, real-world C. edwardii before C. armbrusteri statement for any pair of taxa, the hypothesized event sequence must replicate it. Then, the program shuffles the events using a maximum likelihood criterion. The criterion basically seeks to pull apart as many hypothesized age range overlaps as possible, especially if they involve common taxa. Taxa are defined as "common" if they are known to overlap with a large fraction of the taxa with which they are implied to overlap.
=== Step 3 ===
Once the relative event sequence has been established, it is converted into numerical time with a nonlinear interpolation algorithm that compares event sequence positions and geochronological age estimates for collections that have them. The calibration only uses:
40Argon/39Argon dates
Uranium-thorium dates for some Pleistocene collections
Paleomagnetic dates that derive from unambiguous, narrow correlations inferred using nonfaunal tie points such as the position in the section of the Cretaceous–Paleogene boundary, Paleocene-Eocene boundary, or Recent
== NALMA vs. AEO ==
The North American land mammal ages procedure uses subjective opinions by published sources and/or authors, citing authors such as Michael O. Woodburne, Robert W. Wilson, and J. David Archibald.
Appearance event ordination uses objective, explicit, recordable, repeatable, and quantitative analyses of faunal and biostratigraphic data to arrive at a conclusion, according to John Alroy.
== References ==

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Archival research is a type of research which involves seeking out and extracting evidence from archival records. These records may be held either in collecting institutions, such as libraries and museums, or in the custody of the organization (whether a government body, business, family, or other agency) that originally generated or accumulated them, or in that of a successor body (transferring, or in-house archives). Archival research can be contrasted with (1) secondary research (undertaken in a library or online), which involves identifying and consulting secondary sources relating to the topic of enquiry; and (2) with other types of primary research and empirical investigation such as fieldwork and experiment.
== History of archives organizations ==
The oldest archives have been in existence for hundreds of years. For instance, in Europe, the General Archive of the Crown of Aragon was instituted in 1318, or the Vatican Secret Archives which were started in the 17th century and contain state papers, papal account books, and papal correspondence dating back to the 8th century. The Archives Nationales in France was founded in 1790 during the French Revolution and has holdings that date back to AD 625, and other European archives have a similar provenance. Archives in the modern world, while of more recent date, may also hold material going back several centuries, for example, the United States National Archives and Records Administration was established originally in 1934. The NARA contains records and collections dating back to the founding of the United States in the 18th century. Among the collections of the NARA are the Declaration of Independence, the Constitution of the United States, and an original copy of Magna Carta. The British National Archives (TNA) traces its history to the creation of the Public Record Office in 1838, while other state and national bodies were also formed in the late 19th and early 20th centuries.
Universities are another venue for archival holdings and manuscript collections. Most universities have archival holdings that chronicle the business of the university. Some universities also have archives or manuscript collections that focus on one aspect or another of the culture of the state or country in which the university is located. Schools and religious institutions, as well as local studies and history collections, museums and research institutions may all hold archives.
The reason for highlighting the breadth and depth of archives is to give some idea of the difficulties facing archival researchers. Some of these archives hold vast quantities of records. For example, the Vatican Secret Archive has upwards of 52 miles of archival shelving. An increasing number of archives are now accepting digital transfers, which can also present challenges for display and access.
== Archival research methodologies ==
Archival research lies at the heart of most academic and other forms of original historical research; but it is frequently also undertaken (in conjunction with parallel research methodologies) in other disciplines within the humanities and social sciences, including literary studies, rhetoric, archaeology, sociology, human geography, anthropology, psychology, and organizational studies. It may also be important in other non-academic types of enquiry, such as the tracing of birth families by adoptees, and criminal investigations. Data held by archival institutions is also of use in scientific research and in establishing civil rights.
In addition to discipline, the kind of research methodology used in archival research can vary depending on its organization and its materials. For example, in an archives that has a large number of materials still unprocessed, a researcher may find consulting directly with archive staff who have a clear understanding of collections and their organization to be useful as they can be a source of information regarding unprocessed materials or of related materials in other archives and repositories. When an archive is not entirely oriented towards one or relevant to a single discipline, researchers, for example genealogists, may rely upon formal or informal networks to support research by sharing information about specific archives' organization and collections with each other.
== Conducting research at an archive ==
Archival research is generally more complex and time-consuming than secondary research, presenting challenges in identifying, locating and interpreting relevant documents. Although archives share similar features and characteristics they can also vary in significant ways. While publicly funded archives may have mandates that require them to be as accessible as possible, other kinds, such as corporate, religious, or private archives, will have varying degrees of access and discoverability. Some materials may be restricted in other ways, such as on those containing sensitive or classified information, unpublished works, or imposed by agreements with the donor of materials. Furthermore, archival records are often unique, and the researcher must be prepared to travel to reach them. Even when materials are available in digital formats there may be restrictions on them that prohibit them from being accessed off-site.

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=== Locating archival collections ===
Prior to online search, union catalogs were an important tool for finding materials in libraries and archives. In the United States, the National Union Catalog and the National Union Catalog of Manuscript Collections have been used by researchers to locate archives although much of its information has since been migrated to online systems.
An increasing number of archival institutions can be found via an online search. In addition, portals such as Europeana, the Digital Public Library of America and the National Library of Australia's Trove provide links to member institutions.
In the UK, JISC hosts the ArchivesHub, while the OCLC's ArchiveGrid provides an international portal for mostly library based institutions, which use MARC as a cataloguing tool for their holdings. The Association of Canadian Archivists (ACA) has partnered with the software company Artefactual to create ArchivesCanada, while the Australian Society of Archivists have used the same software for their Directory of Archives in Australia. Many other online search tools have been made available to facilitate search and discovery, including the Location Register of English Literary Manuscripts and Letters, the ArchiveSearch guide to archival materials in institutions in Cambridge, UK, and CARTOMAC: Archives littéraires d'Afrique.
If an archives cannot be found through online search or a publicly listed collection a researcher may have to track down its existence through other means, such as following other researcher's citations and references. This is particularly true for materials held by corporations or other organizations that may not employ an archivist and thus be unaware of the extent or contents of their materials.
In very restricted archives, access may be restricted only to individuals with certain credentials or affiliations with institutions like universities and then only to those of a certain level. Those lacking the necessary credentials may need to request letters of introduction from an individual or institution to provide to the archive.
=== Locating materials within archives ===
Archives usually contain unique materials and their organization may also be entirely unique or idiosyncratic to the institution or organization that maintains them. This is one important distinction with libraries where material is organized according to standardized classification systems. Traditionally, archives have followed the principle of respect des fonds in which the provenance and original order is maintained although some rearrangement, physical or intellectual, may be done by the archivist to facilitate its use. A basic guideline for archival description is the International Standard of Archival Description (General) (ISAD/G or ISAD), produced by the International Council on Archives (ICA). American institutions may also be guided by Describing Archives: a content standard (DACS) and in Canada by the Rules of Archival Description Archived 16 May 2017 at the Wayback Machine (RAD). Understanding how archival descriptions and finding aids are constructed is known as archival intelligence.
In addition to these standards and rules for creating hard copy and online listings and catalogues, archivists may also provide access to their catalogues through APIs or through the encoding standards EAD (Encoded archival description) (relating to the fonds, series, and items) and EAC (Encoded archival context)(the organisations and people that created the archives).
Finding aids are a common reference tool created by archivists for locating materials. They come in a variety of forms, such as registers, card catalogs, or inventories. Many finding aids to archival documents are now hosted online as web pages or uploaded as documents, such as at the Library of Congress' Rare Book & Special Collections. The level of detail in finding aids can vary from granular item-level descriptions to coarse collection-level descriptions. If an archive has a large backlog of unprocessed materials, there may not be any kind of finding aid at all. From around 2005, an ideology known as "More Product, Less Process", or MPLP, has been adopted by many North American collecting archives seeking to reduce processing time or alleviate backlogs to provide access to materials sooner, the results of which may be minimally described finding aids.
Although most archive repositories welcome researchers, and have professional staff tasked with assisting them, the large quantity of records means that finding aids may be of only limited usefulness: the researcher will need to hunt through large quantities of documents in search of material relevant to his or her particular enquiry. Some records may be closed to public access for reasons of confidentiality; and others may be written in archaic handwriting, in ancient or foreign languages, or in technical terminology. Archival documents were generally created for immediate practical or administrative purposes, not for the benefit of future researchers, and additional contextual research may be necessary to make sense of them. Many of these challenges are exacerbated when the records are still in the custody of the generating body or in private hands, where owners or custodians may be unwilling to provide access to external enquirers, and where finding aids may be even more rudimentary or non-existent.
=== Consulting archival materials ===
==== On-site ====
Archival materials are usually held in closed stacks and non-circulating. Users request to see specific materials from the archives and may only consult them on-site. After locating the relevant record location using a finding aid or other discovery tool a user may then have to submit the request to the archives, such as using a request form. If an archives has part of its holdings located in a separate building or facility, it make take days or weeks to retrieve materials, requiring a user to submit their requests in advance of an on-site consultation.

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A reading room is a space, usually within or near the archive, where users can consult archival materials under staff supervision. The unique, fragile, or sensitive nature of some materials sometimes requires the certain kinds of restrictions on their use, handling, and/or duplication. Many archives restrict what kinds of items can be brought into a reading room from outside, such as pencils, notepads, bags, and even clothing, to guard against theft or risk of damage to materials. Further restrictions may be placed on the number of materials that can be consulted at any given time, such as limiting a user to one box at a time and requiring all materials to be laid flat and visible at all times. Some archives provide basic supplies including scrap paper and pencils or foam wedges for supporting unusually large materials. Duplication services may be available at the archive although the policies, costs, and time required can vary. Increasingly, archives also allow users to use their own devices, such as handheld cameras, cell phones, and even scanners, to duplicate materials. The use of white or any other glove, while popular in television programs, is not necessarily required for handling archival documents, due to concerns about fragility of pages and text. They may be required for handling volumes with poor bindings, if the gloves are removed for the internal pages to prevent transfer of dirt and other material, and should be used when handling photographs. Always check with the archivist as to whether gloves are required or not.
Archives may also provide access to content via microfilm (including fiche and other formats) due to the fragility or popularity of the original archive. Digital copies may also be provided for the same reason. Before asking for access to the original, users should make sure that the items that have been reformatted are suitable for the use for which they are required. Reasons for asking for access to original content might include the need to view a colour image (architectural perspective and elevation drawings, maps and plans, etc.) or for accessibility reasons (minor visual vertigo is usually not considered a reason for access to originals, as the effect can be mitigated by slower perusal of the film).
Some materials may contain information that concerns the privacy and confidentiality of living individuals, such as medical and student records, and demand special care. Materials that might contain personally identifiable information, such as social security numbers or names, must be handled appropriately, and an archive might provide redacted copies of materials or deny access to materials entirely due to privacy or other legislative concerns.
==== Off-site and electronic materials ====
More and more archival materials are being digitized or are born-digital enabling them to be accessed off-site through the internet or other networked services. Archives that have digital materials accessible to the public may make their holdings discoverable to internet search engines by sharing or exposing their electronic catalogs and/or metadata, using standards like the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH). Some institutions have online portals where users can freely access digital materials that have been made available by the archive such as the Archives of the New York Public Library or the Smithsonian Institution Archives. Governments and their related institutions may use these "electronic", or "virtual", reading rooms to upload documents and materials that have been requested by the public such as through FOIA requests or in accordance with records disclosure policies.
== References ==
== External links ==
National Archives and Records Administration (NARA), United States of America
NARA: "Research Our Records"
The National Archives (TNA), United Kingdom
TNA: "Help with your research"
TNA: "How to use archives"
Trace Your Birth Family In The UK
"Archive skills and tools for historians" - Making History (Institute of Historical Research, University of London)
Society of American Archivists: Using Archives: A Guide to Effective Research
=== LibGuides on Archival Research ===
Guide to Archival Research (Dalhouse University)
Archival Research Guide (Georgetown University Library)
A Guide to Archival Research (Emory Libraries)
Introduction to Archival Research (Duke University Libraries)
Archival Research: Why Archival Research (Georgia State University)
Doing Archival Research (Williams College)
Conducting Archival Research (University of the Witwatersrand)

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Astronomical chronology, or astronomical dating, is a technical method of dating events or artifacts that are associated with astronomical phenomena. Written records of historical events that include descriptions of astronomical phenomena have done much to clarify the chronology of the Ancient Near East; works of art which depict the configuration of the stars and planets and buildings which are oriented to the rising and setting of celestial bodies at a particular time have all been dated through astronomical calculations.
== Dating historical events ==
The use of descriptions of astronomical phenomena to date historical events began in the 16th century, a time of a renewed humanistic interest in history and of increasingly precise astronomical tables. Eclipses in particular are relatively infrequent events and can be dated precisely. When the circumstances are not exact and descriptions leave ambiguities, one can often use other details such as the month of the eclipse or the position of other stars and planets to identify the specific eclipse.
Astronomical dating, like other forms of historical interpretation, requires care in interpreting the surviving written records. John Steele has proposed three questions that must be asked when dating an event: Does the record refer to an actual astronomical event, or is this merely a modern assumption? If it does refer to an actual astronomical event, is the source reliable? Can the record provide an unambiguous date without making unwarranted assumptions about ancient astronomical observational methods?
Babylonian astronomical diaries provide detailed and unambiguous accounts of the positions of all the visible planets, often in relation to specific stars, that have been used to provide precise dates of events like the defeat of Darius III by Alexander the Great at the Battle of Gaugamela on 1 October 331 BCE and of Alexander's subsequent death on 11 June 323.
Since the success of this method depends on the reliability of the written sources and the precision of their accounts of astronomical phenomena, attempts to date literary texts which may describe astronomical events loosely or even as metaphors have led researchers to conclusions that appear precise, but rely on invalid assumptions and are consequently less widely accepted. Thus the attempts to date Vedic texts describing the Pleiades as rising "due East" to about 2300 BCE, which is the time when the Pleiades rose "exactly" due East, is complicated by the fact that poetic descriptions need not be taken as reflecting precise astronomical observations, while precession is a very slow process which makes only small changes in the azimuth of a star rising in the East.
== Dating artifacts ==
Among the artifacts that can most readily be dated by astronomical techniques are depictions of the positions of the celestial bodies at a particular time. Since the motions of the celestial bodies are all at different periods, it takes many centuries for all the planets plus the Sun and the Moon, to reach the same positions in the signs of the Zodiac. For a configuration accurate to ±15° (that is, within a single sign) the positions of these seven bodies will only return to the same configuration once in about 3700 years. A particular case involved a medieval illuminated manuscript which portrayed the position of these seven celestial bodies on 18 March 816; corresponding to the period when the manuscript was written. This calculation demonstrated that this illustration was not a copy of an earlier classical depiction of the position of the stars. The rapidly moving Moon is the most sensitive indicator for the exact time; if one can estimate the indicated position of the Moon to within a degree, the time of the diagram can be computed to within an hour.
A striking example of this method was an astrological portrait of Sir Christopher Hatton (1540–1591), which depicted the positions of the seven classical planets in the zodiac and noted the computed positions of the planets to the nearest minute of arc. Here the largest source of error in the date was the uncertainty of 16th-century astronomical calculations. The resulting time was about noon of 12 December 1581.
== Dating structures by their orientation ==
A more controversial archaeoastronomical approach has been used to date structures that are believed to have been oriented on astronomical principles by measuring their orientation and computing the date in the past when a single specified celestial body, whether the Sun or a selected star, rises or sets at the measured azimuth. The astronomer Norman Lockyer applied this method to Stonehenge by measuring the orientation of the Stonehenge avenue and comparing it to the position of solstitial sunrise, which changes slowly due to the changing obliquity of the ecliptic. The archaeologist F. C. Penrose applied a similar method to ancient Greek Temples, attempting to establish their dates by relating their orientation to the appearance of stars on the horizon, the position of which changes slowly due to the precession of the equinoxes.
The wide variance of these dates from historically accepted ones led the architect and archaeologist William Bell Dinsmoor to mistrust dates established by the slowly changing obliquity of the ecliptic or by stellar alignments, which involve an arbitrary selection of a star that rises on the proper azimuth. Instead he proposed a method employing what was already known from historical records concerning the dates of construction of Greek temples, the festivals associated with specific temples, and the nature of the Greek Lunisolar calendar. Since the date of a festival in the Greek lunisolar calendar only recurs on the same date in the solar calendar every eight or nineteen years, Dinsmoor identified a festival connected with a specific temple and was able to determine the exact year near the historically recorded construction date when the Sun rose in alignment with the temple on the date of the festival.
== See also ==
Astronomical chronology
Age of the Earth
Age of the universe
Chronological dating, archaeological chronology
Absolute dating
Relative dating
Phase (archaeology)
Archaeological association
Geochronology
Geologic time scale
Geological history of Earth
== Notes ==
== References ==
Neugebaer, Otto. A History of Ancient Mathematical Astronomy, (3 vols). New York: Springer, 1975. Vol. 3, pp. 1071–1076 provides a brief introduction to astronomical chronology.
== Bibliography ==
Fraser, Gordon Fraser. Star Territory: Printing the Universe in Nineteenth-Century America. Material Texts. Philadelphia: University of Pennsylvania Press, 2021.
Giovannetti-Singh, Gianamar. "Astronomical Chronology, the Jesuit China Mission, and Enlightenment History". Journal of the History of Ideas, 83(3) (2023): 487-510. https://doi.org/10.1353/jhi.2023.a901491
Gingerich, Owen and Barbara Welther. Planetary, Lunar, and Solar Positions, A. D. 1650 to 1805, Memoirs of the American Philosophical Society, 59S. Philadelphia, 1983.
Neugebauer, Paul V. Astronomische Chronologie (2 vols). Berlin: De Gruyter, 1929.
Steele, John M. "The Use and Abuse of Astronomy in Establishing Absolute Chronologies", Physics in Canada/La Physique au Canada, 59 (2003): 243-248.
Tuckerman, Bryant. Planetary, Lunar, and Solar Positions, 601 B.C. to A, D. 1, Memoirs of the American Philosophical Society, 56. Philadelphia, 1962.
Tuckerman, Bryant. Planetary, Lunar, and Solar Positions, A. D. 2 to 1649, Memoirs of the American Philosophical Society, 59. Philadelphia, 1964.
== External links ==
van Gent, R.H., Astronomical Chronology

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An attestation is something that serves to bear witness, confirm, authenticate or verify the validity of some fact or status. An attestor is someone who performs an attestation. An attestation date is the date on which an attestation is performed.
== Examples ==
Examples of attestations include:
Testimony, a sworn verification of the truth of a set of factual statements
An attestation clause, verifying a document
A police oath or an oath of allegiance in armed forces of the United Kingdom, pledging loyalty or the faithful execution of duties
A validation of the integrity of a computing device such as a server needed for trusted computing
== See also ==
The dictionary definition of attest at Wiktionary
The dictionary definition of attestation at Wiktionary
The dictionary definition of attestor at Wiktionary
Attested language, a language for which documented evidence exists
== References ==

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Bataan Rice Enrichment Project or the Bataan Experiment, was a collaborative research venture between American chemist Robert R. Williams and Juan Salcedo Jr. It was a series of feeding experiments conducted in municipalities in Bataan between 1947 and 1949. By the end of the experiments, it is shown that thiamine-enriched rice can reduce the cases of beriberi in the Philippines, which was the leading cause of deaths during those times.
== Overview ==
The enrichment project came first as a plan in 1943. During this time, Salcedo, who had his studies at Columbia University, met American chemist Robert R. Williams, a well renowned scientist for his synthesis on vitamin B1 in 1935.
The Philippine Bureau of Health reported a relatively stable beriberi rate from the mid-1920s to 1940. However, after World War II, beriberi cases surged, becoming the second leading cause of death in 1946 and 1947. Infants accounted for a significant portion of these deaths.
At this time, Williams was disappointed by the agencies at the United Nations to further eradicate the rise of beriberi cases around the globe. Due to his previous failures in rice enrichment programs, Williams became desperate. Together with Salcedo, they began feeding experiments in the province of Bataan.
The specific objectives of the feeding experiment were:
Determine if enriched rice could effectively treat beriberi.
Test the practicality of using enriched rice in the rice trade.
Establish a system to ensure only enriched rice is sold.
Promote the use of enriched rice among the people and explore its potential for widespread use throughout the Philippines.
The experiments were conducted in Bataan where it was divided into two areas: the experimental zone and control zone. After the introduction of thiamine-enriched rice, the experimental area received significant results. Mortality rate in beriberi significantly decreased after the introduction of the nutrient-enriched white rice from July 1, 1948, to June 30, 1950. Before the introduction, there occurred 167 deaths from beriberi cases from July 1, 1947, to June 30, 1948. It decreased further to just 18 deaths after the introduction.
== Human rights concerns ==
Williams intentionally exposed half of Bataan's food-deficient population to beriberi, replicating the unethical experiments conducted by Euro-American researchers on prisoners and asylum patients. He also recreated prison camps and asylums to further persuade unwilling participants. This act, unfortunately, was seen as a form of colonial exploitation by both the Filipino people and nationalist physicians, who recognized beriberi as a symptom of the colonial system.
The control group used for the feeding experiments were also denied of access from the enriched rice. This resulted an unwanted exposure of beriberi among research participants.
== Reception and aftermath ==
Due to positive results from the experiment, in 1950, both Williams and Salcedo planned to expand the rice enrichment project throughout the Philippines. However, this was met by opposition from the Philippine government despite insistence from Williams. During the 1950s, it was further delayed by the Hukbalahap rebellion.
From then on, the government stopped subsidizing the project entirely and had to rely from funding by Williams and supported by a team from the Food and Agriculture Organization (FAO). From 1966 to 1970, FAO sponsored the introduction of high-yielding rice varieties into the Philippines. According to a 1971 report by FAO, the implementation of the rice enrichment projects in the Philippines, Taiwan, and Japan showed the "conflicting economic interests of millers,
governments and consumers".
== References ==

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Technically, all scientific experiments measure a change in hypothesized causal behavior, and may drop the behavioral prefix.
Behavioral experiment may refer to:
Behavioral experiment (analysis)
Behavioral experiment (animals), for controlling variables (vs. field studies)
Behavioral experiment (cognitive science), for determining what constitutes intelligent behavior
Behavioral experiment (cognitive therapy), method for cognitive restructuring
Behavioral experiment (cognitive behavioral therapy), for testing the validity of negative and alternative thoughts in real-life situations
Behavioral experiment (computational modeling), of computational model for comparison with human data
Behavioral experiment (experimental psychology), for measuring reaction time, choices among alternatives, and/or response rate or strength
Behavioral experiment (human reasoning), for studying human reasoning
Behavioral experiment (conditional reasoning), on conditionals in the psychology of reasoning
Behavioral experiment (psychotherapy), for identifying potentially negative or harmful beliefs
== See also ==
Behavioral experiments for monotropism
Behaviorism, which is based on such experiments
Experiment
Category:Science experiments
All pages with titles containing experiment
Behavior
All pages with titles containing behavior
All pages with titles containing behaviour

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The burden of proof (Latin: onus probandi, shortened from Onus probandi incumbit ei qui dicit, non ei qui negat – the burden of proof lies with the one who speaks, not the one who denies) is the obligation on a party in a dispute to provide sufficient warrant for its position.
== Holder of the burden ==
When two parties are in a discussion and one makes a claim that the other disputes, the one who makes the claim typically has a burden of proof to justify or substantiate that claim, especially when it challenges a perceived status quo. This is also stated in Hitchens's razor, which declares that "what may be asserted without evidence may be dismissed without evidence." Carl Sagan proposed a related criterion: "Extraordinary claims require extraordinary evidence".
While certain kinds of arguments, such as logical syllogisms, require mathematical or strictly logical proofs, the standard for evidence to meet the burden of proof is usually determined by context and community standards and conventions.
Philosophical debate can devolve into arguing about who has the burden of proof about a particular claim. This has been described as "burden tennis" or the "onus game".
== Shifting the burden of proof ==
One way in which one would attempt to shift the burden of proof is by committing a logical fallacy known as the argument from ignorance. It occurs when either a proposition is assumed to be true because it has not yet been proven false or a proposition is assumed to be false because it has not yet been proven true.
== Proving a negative ==
A negative claim is the opposite of an affirmative or positive claim. It asserts the non-existence or exclusion of something.
Logicians and philosophers of logic reject the notion that it is intrinsically impossible to prove negative claims. Philosophers Steven D. Hale and Stephen Law state that the phrase "you cannot prove a negative" is itself a negative claim that would not be true if it could be proven true. Many negative claims can be rewritten into logically equivalent positive claims (for example, "No Jewish person was at the party" is logically equivalent to "Everyone at the party was a gentile"). In formal logic and mathematics, the negation of a proposition can be proven using procedures such as modus tollens and reductio ad absurdum. In empirical contexts (such as evaluating the existence or nonexistence of unicorns), inductive reasoning is often used for establishing the plausibility of a claim based on observed evidence. Though inductive reasoning may not provide absolute certainty about negative claims, this is only due to the nature of inductive reasoning; inductive reasoning provides proof from probability rather than certainty. Inductive reasoning also does not provide absolute certainty about positive claims.
A negative claim may or may not exist as a counterpoint to a previous claim. A proof of impossibility or an evidence of absence argument are typical methods to fulfill the burden of proof for a negative claim.
== Application ==
=== In public discourse ===
Burden of proof is an important concept in the public arena of ideas. Once participants in discourse establish common assumptions, the mechanism of burden of proof helps to ensure that all parties contribute productively, using relevant arguments.
=== In law ===
In a legal dispute, one party is initially presumed to be correct and gets the benefit of the doubt, while the other side bears the burden of proof. When a party bearing the burden of proof meets their burden, the burden of proof switches to the other side. Burdens may be of different kinds for each party, in different phases of litigation. The burden of production is a minimal burden to produce at least enough evidence for the trier of fact to consider a disputed claim. After litigants have met the burden of production and their claim is being considered by a trier of fact, they have the burden of persuasion, that enough evidence has been presented to persuade the trier of fact that their side is correct. There are different standards of persuasiveness ranging from a preponderance of the evidence, where there is just enough evidence to tip the balance, to proof beyond a reasonable doubt, as in United States criminal courts.
The burden of proof is usually on the person who brings a claim in a dispute. It is often associated with the Latin maxim semper necessitas probandi incumbit ei qui agit, a translation of which in this context is: "the necessity of proof always lies with the person who lays charges."
The party that does not carry the burden of proof carries the benefit of assumption of being correct, they are presumed to be correct, until the burden shifts after presentation of evidence by the party bringing the action. An example is in an American criminal case, where there is a presumption of innocence by the defendant. Fulfilling the burden of proof effectively captures the benefit of assumption, passing the burden of proof off to another party.
=== In statistics ===

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In inferential statistics, the null hypothesis is a general statement or default position that there is no relationship between two measured phenomena, or no association among groups. Rejecting or disproving the null hypothesis—and thus concluding that there are grounds for believing that there is a relationship between two phenomena (e.g. that a potential treatment has a measurable effect)—is a central task in the modern practice of science; the field of statistics gives precise criteria for rejecting a null hypothesis.
The null hypothesis is generally assumed to be true until evidence indicates otherwise. In statistics, it is often denoted H0 (read "H-nought", "H-null", "H-oh", or "H-zero").
The concept of a null hypothesis is used differently in two approaches to statistical inference. In the significance testing approach of Ronald Fisher, a null hypothesis is rejected if the observed data are significantly unlikely to have occurred if the null hypothesis were true. In this case the null hypothesis is rejected and an alternative hypothesis is accepted in its place. If the data are consistent with the null hypothesis, then the null hypothesis is not rejected. In neither case is the null hypothesis or its alternative proven; the null hypothesis is tested with data and a decision is made based on how likely or unlikely the data are. This is analogous to the legal principle of presumption of innocence, in which a suspect or defendant is assumed to be innocent (null is not rejected) until proven guilty (null is rejected) beyond a reasonable doubt (to a statistically significant degree).
In the hypothesis testing approach of Jerzy Neyman and Egon Pearson, a null hypothesis is contrasted with an alternative hypothesis and the two hypotheses are distinguished on the basis of data, with certain error rates.
Proponents of each approach criticize the other approach. Nowadays, though, a hybrid approach is widely practiced and presented in textbooks. The hybrid is in turn criticized as incorrect and incoherent—for details, see Statistical hypothesis testing.
Statistical inference can be done without a null hypothesis, by specifying a statistical model corresponding to each candidate hypothesis and using model selection techniques to choose the most appropriate model. (The most common selection techniques are based on either Akaike information criterion or Bayes factor.)
== See also ==
== References ==

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Cherry picking, suppressing evidence, or the fallacy of incomplete evidence is the act of pointing to individual cases or data that seem to confirm a particular position while ignoring a significant portion of related and similar cases or data that may contradict that position. Cherry picking may be committed intentionally or unintentionally.
== Name ==
The term is based on the perceived process of harvesting fruit, such as cherries. The picker would be expected to select only the ripest and healthiest fruits. An observer who sees only the selected fruit may thus wrongly conclude that most, or even all, of the tree's fruit is in a likewise good condition. This can also give a false impression of the quality of the fruit (since it is only a sample and is not a representative sample). A concept sometimes confused with cherry picking is the idea of gathering only the fruit that is easy to harvest, while ignoring other fruit that is higher up on the tree and thus more difficult to obtain (see low-hanging fruit).
Cherry picking has a negative connotation as the practice neglects, overlooks or directly suppresses evidence that could lead to a complete picture.
Cherry picking can be found in many logical fallacies. For example, the "fallacy of anecdotal evidence" tends to overlook large amounts of data in favor of that known personally, "selective use of evidence" rejects material unfavorable to an argument, while a false dichotomy picks only two options when more are available. Some scholars classify cherry-picking as a fallacy of selective attention, the most common example of which is the confirmation bias. Cherry picking can refer to the selection of data or data sets so a study or survey will give desired, predictable results which may be misleading or even completely contrary to reality.
== History ==
A story about the 5th century BCE atheist philosopher Diagoras of Melos says how, when shown the votive gifts of people who had supposedly escaped death by shipwreck by praying to gods, he pointed out that many people had died at sea in spite of their prayers, yet these cases were not likewise commemorated (this is an example of survivorship bias).
Michel de Montaigne (1533–1592) in his essay on prophecies comments on people willing to believe in the validity of supposed seers:
I see some who are mightily given to study and comment upon their almanacs, and produce them to us as an authority when anything has fallen out pat; and, for that matter, it is hardly possible but that these alleged authorities sometimes stumble upon a truth amongst an infinite number of lies. ... I think never the better of them for some such accidental hit. ... [N]obody records their flimflams and false prognostics, forasmuch as they are infinite and common; but if they chop upon one truth, that carries a mighty report, as being rare, incredible, and prodigious.
== In science ==
Cherry picking is one of the epistemological characteristics of denialism and widely used by different science denialists to seemingly contradict scientific findings. For example, it is used in climate change denial, evolution denial by creationists, denial of the negative health effects of consuming tobacco products and of passive smoking. P-hacking may also be considered a form of cherry-picking.
Choosing to make selective choices among competing evidence, so as to emphasize those results that support a given position, while ignoring or dismissing any findings that do not support it, is a practice known as "cherry picking" and is a hallmark of poor science or pseudo-science.
Rigorous science looks at all the evidence (rather than cherry picking only favorable evidence), controls for variables as to identify what is actually working, uses blinded observations so as to minimize the effects of bias, and uses internally consistent logic."
== In medicine ==
In a 2002 study, a review of previous medical data found cherry picking in tests of anti-depression medication:
[researchers] reviewed 31 antidepressant efficacy trials to identify the primary exclusion criteria used in determining eligibility for participation. Their findings suggest that patients in current antidepressant trials represent only a minority of patients treated in routine clinical practice for depression. Excluding potential clinical trial subjects with certain profiles means that the ability to generalize the results of antidepressant efficacy trials lacks empirical support, according to the authors.
== In argumentation ==
In argumentation, the practice of "quote mining" is a form of cherry picking, in which the debater selectively picks some quotes supporting a position (or exaggerating an opposing position) while ignoring those that moderate the original quote or put it into a different context. Cherry picking in debates is a large problem as the facts themselves are true but need to be put in context. Because research cannot be done live and is often untimely, cherry-picked facts or quotes usually stick in the public mainstream and, even when corrected, lead to widespread misrepresentation of the groups targeted.
=== One-sided argument ===
A one-sided argument (also known as card stacking, stacking the deck, ignoring the counter-evidence, slanting, and suppressed evidence) is an informal fallacy that occurs when only the reasons supporting a proposition are supplied, while all reasons opposing it are omitted.
Philosophy professor Peter Suber has written:The one-sidedness fallacy does not make an argument invalid. It may not even make the argument unsound. The fallacy consists in persuading readers, and perhaps ourselves, that we have said enough to tilt the scale of evidence and therefore enough to justify a judgment. If we have been one-sided, though, then we haven't yet said enough to justify a judgment. The arguments on the other side may be stronger than our own. We won't know until we examine them.
So the one-sidedness fallacy doesn't mean that your premises are false or irrelevant, only that they are incomplete.

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[…] You might think that one-sidedness is actually desirable when your goal is winning rather than discovering a complex and nuanced truth. If this is true, then it's true of every fallacy. If winning is persuading a decision-maker, then any kind of manipulation or deception that actually works is desirable. But in fact, while winning may sometimes be served by one-sidedness, it is usually better served by two-sidedness. If your argument (say) in court is one-sided, then you are likely to be surprised by a strong counter-argument for which you are unprepared. The lesson is to cultivate two-sidedness in your thinking about any issue. Beware of any job that requires you to truncate your own understanding.
Card stacking is a propaganda technique that seeks to manipulate audience perception of an issue by emphasizing one side and repressing another. Such emphasis may be achieved through media bias or the use of one-sided testimonials, or by simply censoring the voices of critics. The technique is commonly used in speeches by political candidates to discredit their opponents and to make themselves seem more worthy.
The term originates from the magician's gimmick of "stacking the deck", which involves presenting a deck of cards that appears to have been randomly shuffled but which is, in fact, 'stacked' in a specific order. The magician knows the order and is able to control the outcome of the trick. In poker, cards can be stacked so that certain hands are dealt to certain players.
The phenomenon can be applied to any subject and has wide applications. Wherever a broad spectrum of information exists, appearances can be influenced by highlighting some facts and ignoring others. Card stacking can be a tool of advocacy groups or of those groups with specific agendas. For example, an enlistment poster might focus upon an impressive picture, with words such as "travel" and "adventure", while placing the words, "enlist for two to four years" at the bottom in a smaller and less noticeable font size.
== See also ==
== References ==

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Chronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously established chronology. This usually requires what is commonly known as a "dating method". Several dating methods exist, depending on different criteria and techniques, and some very well known examples of disciplines using such techniques are, for example, history, geology, paleontology, archaeology, astronomy and even forensic science, since in the latter it is sometimes necessary to investigate the moment in the past during which the death of a cadaver occurred. These methods are typically identified as absolute, which involves a specified date or date range, or relative, which refers to dating which places artifacts or events on a timeline relative to other events and/or artifacts. Other markers can help place an artifact or event in a chronology, such as nearby writings and stratigraphic markers.
== Absolute and relative dating ==
Dating methods are most commonly classified following two criteria: relative dating and absolute dating.
=== Relative dating ===
Relative dating methods are unable to determine the absolute age of an object or event, but can determine the impossibility of a particular event happening before or after another event of which the absolute date is well known. In this relative dating method, Latin terms ante quem and post quem are usually used to indicate both the most recent and the oldest possible moments when an event occurred or an artifact was left in a stratum, respectively. But this method is also useful in many other disciplines. Historians, for example, know that Shakespeare's play Henry V was not written before 1587 because Shakespeare's primary source for writing his play was the second edition of Raphael Holinshed's Chronicles, not published until 1587. Thus, 1587 is the post quem dating of Shakespeare's play Henry V. That means that the play was without fail written after (in Latin, post) 1587.
The same inductive mechanism is applied in archaeology, geology and paleontology, by many ways. For example, in a stratum presenting difficulties or ambiguities to absolute dating, paleopalynology can be used as a relative referent by means of the study of the pollens found in the stratum. This is admitted because of the simple reason that some botanical species, whether extinct or not, are well known as belonging to a determined position in the scale of time.
For a non-exhaustive list of relative dating methods and relative dating applications used in geology, paleontology or archaeology, see the following:
Cross-cutting relationships
Fluorine absorption dating
Harris matrix
Law of included fragments
Law of superposition
Lichenometry
Marine isotope stages, based on the oxygen isotope ratio cycle
Melt inclusions
Morphology (archaeology)
Nitrogen dating
Palynology, the study of modern-dated pollens for the relative dating of archaeological strata, also used in forensic palynology.
Paleomagnetism
Paleopalynology, also spelt "Palaeopalynology", the study of fossilized pollens for the relative dating of geological strata.
Principle of original horizontality
Principle of lateral continuity
Principle of faunal succession
Seriation (archaeology)
Sequence dating (a type of seriation)
Tephrochronology
Typology (archaeology)
Uranium–lead dating. Lead corrosion dating (exclusively used in archaeology)
Varnish microlamination
Vole clock
=== Absolute dating ===
Absolute dating methods seek to establish a specific time during which an object originated or an event took place. While the results of these techniques are largely accepted within the scientific community, there are several factors which can hinder the discovery of accurate absolute dating, including sampling errors and geological disruptions. This type of chronological dating utilizes absolute referent criteria, mainly the radiometric dating methods. Material remains can be absolutely dated by studying the organic materials which construct the remains. For example, remains that have pieces of brick can undergo the process of thermoluminescence (TL) dating in order to determine approximately how many years ago the material was fired. This technique was used to discover the date of St. James Church in Toruń by testing the thermoluminescence of removed bricks. In this example, an absolute date was determined which filled a gap in the historical knowledge of the church.
These techniques are utilized in many other fields as well. Geologists, for example, apply absolute dating methods to rock sediment in order to discover their period of origin.
Some examples of both radiometric and non-radiometric absolute dating methods are the following:
Amino acid dating
Archaeomagnetic dating
Argon–argon dating
Astronomical chronology
Carbon dating: Also known as radiocarbon dating, it can reveal the age of organic material in artifacts as well as human and animal remains. This process can reliably measures dates up to approximately 50,000 years ago.
Cementochronology, this method does not determine a precise moment in a scale of time but the age at death of a dead individual.
Datestone (exclusively used in archaeology)
Dendrochronology
Electron spin resonance dating
Fission track dating
Geochronology
Herbchronology
Iodine–xenon dating
Potassium–argon dating
Lead–lead dating
Luminescence dating
Thermoluminescence dating
Optically stimulated luminescence
Optically stimulated luminescence thermochronometry
Molecular clock (used mostly in phylogenetics and evolutionary biology)
Obsidian hydration dating (exclusively used in archaeology)
Oxidizable carbon ratio dating
Rehydroxylation dating
Rubidium–strontium dating
Samarium–neodymium dating
Tephrochronology
Uranium–lead dating
Uranium–thorium dating
Uranium–uranium dating, useful in dating samples between about 10,000 and 2 million years Before Present (BP), or up to about eight times the half-life of 234U.
Wiggle matching

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== Dating methods in archaeology ==
Just like geologists or paleontologists, archaeologists are also brought to determine the age of both ancient and recent humans. Thus, to be considered as archaeological, the remains, objects or artifacts to be dated must be related to human activity. It is commonly assumed that if the remains or elements to be dated are older than the human species, the disciplines which study them are sciences such geology or paleontology, among some others.
Nevertheless, the range of time within archaeological dating can be enormous compared to the average lifespan of a singular human being. As an example Pinnacle Point's caves, in the southern coast of South Africa, provided evidence that marine resources (shellfish) have been regularly exploited by humans as of 170,000 years ago. On the other hand, remains as recent as a hundred years old can also be the target of archaeological dating methods. It was the case of an 18th-century sloop whose excavation was led in South Carolina (United States) in 1992. Thus, from the oldest to the youngest, all archaeological sites are likely to be dated by an appropriate method.
Dating material drawn from the archaeological record can be made by a direct study of an artifact, or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence relative to datable contexts. Dating is carried out mainly post excavation, but to support good practice, some preliminary dating work called "spot dating" is usually run in tandem with excavation. Dating is very important in archaeology for constructing models of the past, as it relies on the integrity of dateable objects and samples. Many disciplines of archaeological science are concerned with dating evidence, but in practice several different dating techniques must be applied in some circumstances, thus dating evidence for much of an archaeological sequence recorded during excavation requires matching information from known absolute or some associated steps, with a careful study of stratigraphic relationships.
In addition, because of its particular relation with past human presence or past human activity, archaeology uses almost all the dating methods that it shares with the other sciences, but with some particular variations, like the following:
=== Written markers ===
Epigraphy – analysis of inscriptions, via identifying graphemes, clarifying their meanings, classifying their uses according to dates and cultural contexts, and drawing conclusions about the writing and the writers.
Numismatics – many coins have the date of their production written on them or their use is specified in the historical record.
Palaeography – the study of ancient writing, including the practice of deciphering, reading, and dating historical manuscripts.
=== Seriation ===
Seriation is a relative dating method (see, above, the list of relative dating methods). An example of a practical application of seriation, is the comparison of the known style of artifacts such as stone tools or pottery.
=== Age-equivalent stratigraphic markers ===
Paleomagnetism (a relative dating method, see the corresponding list above)
Marine isotope stages based on the oxygen isotope ratio cycle (a relative dating method, see the corresponding list above)
Tephrochronology (an absolute dating method, see the corresponding list above)
=== Stratigraphic relationships ===
The stratigraphy of an archaeological site can be used to date, or refine the date, of particular activities ("contexts") on that site. For example, if a context is sealed between two other contexts of known date, it can be inferred that the middle context must date to between those dates.
== See also ==
Astronomical chronology
Age of Earth
Age of the universe
Geochronology
Geologic time scale
Geological history of Earth
Archaeological science
== References ==

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Cloud seeding in the United Arab Emirates is a weather modification technique used by the government to address water challenges in the country. Cloud seeding is also referred to as man made precipitation and artificial rain making. The United Arab Emirates is one of the first countries in the Persian Gulf region to use cloud seeding technology. UAE scientists use cloud seeding technology to supplement the country's water insecurity, which stems from the extremely hot climate. They use weather radars to continuously monitor the atmosphere of the country. Forecasters and scientists have estimated that cloud seeding operations can enhance rainfall by as much as 30-35% percent in a clear atmosphere, and up to 10-15% in a more humid atmosphere. This practice has caused concerns regarding the impact on the environment because it is difficult to predict its long-term global implications.
== Climate needs ==
The UAE has an arid climate with less than 100mm per year of rainfall, a high evaporation rate of surface water and a low groundwater recharge rate. Rainfall in the UAE has been fluctuating over the last few decades in winter season between December and March.
The climate of the UAE is very dry aside from the coast and the border of the UAE and Oman, where there is high humidity. The UAE is located in a dust hotspot that contributes to the arid climate. There is little to no rainfall, due to frontal systems from the west and northwest, which yield few inches of rainfall per year. This lack of rainfall has scientists and the government worried about water security in the future.
Due to industrialization and population growth, the demand for water has rapidly increased. Current resources are being depleted and scarcity issues are arising as climate change increases evaporation rates, causing drought. As a result, the UAE is looking to cloud seeding technologies to increase water security as well as renewability to combat water and food scarcity that may arise. Research has predicted that drought frequencies and temperatures will continue increasing and cloud seeding hopes to provide an additional method of mitigation against future climate change.
== History ==
Scientists have been experimenting with cloud seeding technology since the 1940s. The cloud-seeding program in the UAE was initiated in the late 1990s, as one of the first Middle Eastern countries to utilize this technique. In 2005, the UAE launched the UAE Prize for Excellence in Advancing the Science and Practice of Weather Modification in collaboration with the World Meteorological Organization (WMO). In 2010, cloud seeding began as a project by weather authorities to create artificial rain. The project, which began in July 2010 and cost $11 million USD, succeeded in creating rain storms in the Dubai and Abu Dhabi deserts.
=== Government involvement ===
The UAE government developed a research program called the UAE Research Program for Rain Enhancement Science (UAEREP) in 2015. It allows scientists and researchers to pitch their potential solutions and conduct research to improve the accuracy of cloud seeding technology. After pitching research proposals, scientists are awarded grants through the UAEREP. Among its key goals are advancing the science, technology, and implementation of rain enhancement and encouraging additional investments in research funding and research partnerships to advance the field, increasing rainfall and ensuring water security globally. By early 2001, the UAEREP was conducting research projects in cooperation with the National Center for Atmospheric Research (NCAR) in the U.S., the Witwatersrand University in South Africa, the National Aeronautics and Space Agency (NASA) in the U.S.
The Program for Rain Enhancement Science is an initiative of the United Arab Emirates Ministry of Presidential Affairs. It is overseen by the UAE National Center of Meteorology & Seismology (NCMS) based in Abu Dhabi.
In 2014, a total of 187 missions were sent to seed clouds in the UAE with each aircraft taking about three hours to target five to six clouds at a cost of $3,000 per operation. In 2017, the UAE had 214 missions, and in 2018, it had 184 missions, and 247 missions were launched in 2019. Tests of new technologies were done in 2020 with partners in the United States to test the use of nanomaterials for seeding.
== Technology ==
The augmentation of rainfall considers both the ground-based and airborne processes that occur in different rain cloud types (but generally focused on convective clouds). The UAE utilizes operational aircraft-based and drone-controlled hygroscopic cloud seeding as opposed to conventional randomized aircraft seeding, as it does not take into consideration the varying properties of rain clouds, especially present in dusty and arid regions like the UAE. Since 2021, the devices have been equipped with a payload of electric-charge emission instruments and customized sensors that fly at low altitudes and deliver an electric charge to air molecules. Hygroscopic cloud seeding uses natural salts such as potassium chloride and sodium chloride that pre-exist in the atmosphere with hygroscopic flares. By introducing Hygroscopic particles, it enhances the natural rain particles which begins a collision-coalescence process.
At present, the UAE mostly cloud seeds in the eastern mountains on the border to Oman to raise levels in aquifers and reservoirs. There are 75 networked automatic weather stations distributed across the country, 7 air quality stations, a Doppler weather radar network of five stationary and one mobile radar, and six Beechcraft King Air C90 aircraft distributed across the country for cloud seeding operations.
== Environmental impact ==

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=== Flooding ===
It is predicted that climate change will lead to higher temperatures, increased humidity and a greater risk of flooding in parts of the Gulf region. These issues could be worsened in nations like the UAE which do not have adequate drainage infrastructure to manage heavy rainfall.
Cloud seeding activities conducted in 2019 by the UAE National Center of Meteorology & Seismology (NCM) as part of the UAE Research Program for Rain Enhancement Science were carried out prior to floods in Dubai in 2019. Although the NCM has linked heavier rainfall to cloud seeding operations, they assert it was not the cause of the flooding. Commercial and residential areas were severely impacted and pumps were needed to remove excess water due to inadequate drainage systems because drainage systems could not handle the volume of water. The UAE planned to invest 500 million dirhams ($136.1 million) on flood protection and transport infrastructure after severe storms in 2020.
Sharjah, one of the most populous cities in the UAE, has experienced repetitive urban flooding during the rainy season over the last three decades. Possible additional increased rainfall intensity due to cloud seeding would require additional investment in the city's drainage systems to mitigate flood risk.
==== April 2024 floods ====
Experts are doubtful that cloud seeding played a role in the UAE's April 2024 floods, suggesting that the heavy rainfall was more likely caused by anthropogenic climate change.
=== Atmospheric aerosols ===
Cloud seeding missions require firing salts and silver iodide crystals into the atmosphere. The increased concentration of particulate matter, or micro-pollutants, increases risk for respiratory illnesses. In 2017, a study was conducted before and after cloud seeding missions, which recorded an increase of particulate matter, correlating to the months of active artificial rain. Researchers attribute this to left over silver iodine crystals that were not dispersed in the rain during the cloud seeding months. A study was conducted called the UAE Unified Aerosol Experiment (UAE2) to assess the progress and effectiveness of cloud seeding specifically in the UAE. Researchers found a significant increase in rainfall trends in areas with cloud seeding. More recently, over 20 regions in the UAE that participated in cloud seeding experiments have a higher concentration of particulate matter. The overall environmental impact of cloud seeding is difficult measure due to the inability to perform controlled experiments along with the difficulty in direct tracing.
== See also ==
Cloud seeding
United Arab Emirates
Environmental issues in the United Arab Emirates
Arabian Desert
Abu Dhabi
Dubai Electricity and Water Authority
Sharjah Electricity and Water Authority
Particulates
== References ==

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A control variable (or scientific constant) in scientific experimentation is an experimental element which is constant (controlled) and unchanged throughout the course of the investigation. Control variables could strongly influence experimental results were they not held constant during the experiment in order to test the relative relationship of the dependent variable (DV) and independent variable (IV). The control variables themselves are not of primary interest to the experimenter.
"Good controls", also known as “confounders” or “deconfounders”, are variables which are theorized to be unaffected by the treatment and which are intended to eliminate omitted-variable bias. "Bad controls", on the other hand, are variables that could be affected by the treatment, might contribute to collider bias, and lead to erroneous results.
== Usage ==
A variable in an experiment which is held constant in order to assess the relationship between multiple variables, is a control variable. A control variable is an element that is not changed throughout an experiment because its unchanging state allows better understanding of the relationship between the other variables being tested.
In any system existing in a natural state, many variables may be interdependent, with each affecting the other. Scientific experiments test the relationship of an IV (or independent variable: that element that is manipulated by the experimenter) to the DV (or dependent variable: that element affected by the manipulation of the IV). Any additional independent variable can be a control variable.
A control variable is an experimental condition or element that is kept the same throughout the experiment, and it is not of primary concern in the experiment, nor will it influence the outcome of the experiment. Any unexpected (e.g.: uncontrolled) change in a control variable during an experiment would invalidate the correlation of dependent variables (DV) to the independent variable (IV), thus skewing the results, and invalidating the working hypothesis. This indicates the presence of a spurious relationship existing within experimental parameters. Unexpected results may result from the presence of a confounding variable, thus requiring a re-working of the initial experimental hypothesis. Confounding variables are a threat to the internal validity of an experiment. This situation may be resolved by first identifying the confounding variable and then redesigning the experiment taking that information into consideration. One way to this is to control the confounding variable, thus making it a control variable. If, however, the spurious relationship cannot be identified, the working hypothesis may have to be abandoned.
== Experimental examples ==
Take, for example, the well known combined gas law, which is stated mathematically as:
P
V
T
=
k
{\displaystyle \qquad {\frac {PV}{T}}=k}
where:
P is the pressure
V is the volume
T is the thermodynamic temperature measured in kelvins
k is a constant (with units of energy divided by temperature).
which shows that the ratio between the pressure-volume product and the temperature of a system remains constant.
In an experimental verification of parts of the combined gas law, (P * V = T), where Pressure, Temperature, and Volume are all variables, to test the resultant changes to any of these variables requires at least one to be kept constant. This is in order to see comparable experimental results in the remaining variables.
If Temperature is made the control variable and it is not allowed to change throughout the course of the experiment, the relationship between the dependent variables, Pressure, and Volume, can quickly be established by changing the value for one or the other, and this is Boyle's law. For instance, if the Pressure is raised then the Volume must decrease.
If, however, Volume is made the control variable and it is not allowed to change throughout the course of the experiment, the relationship between dependent variables, Pressure, and Temperature, can quickly be established by changing the value for one or the other, and this is Gay-Lussac's law. For instance, if the Pressure is raised then the Temperature must increase.
== Notes ==
== References ==
== External links ==
Definitions; Science Buddies – Science Fair Projects.

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Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed in a tree. As well as dating them, this can give data for dendroclimatology, the study of climate and atmospheric conditions during different periods in history from the wood of old trees. Dendrochronology derives from the Ancient Greek dendron (δένδρον), meaning "tree", khronos (χρόνος), meaning "time", and -logia (-λογία), "the study of".
Dendrochronology is useful for determining the precise age of samples, especially those that are too recent for radiocarbon dating, which always produces a range rather than an exact date. However, for a precise date of the death of the tree a full sample to the edge is needed, which most trimmed timber will not provide. It also gives data on the timing of events and rates of change in the environment (most prominently climate) and also in wood found in archaeology or works of art and architecture, such as old panel paintings. It is also used as a check in radiocarbon dating to calibrate radiocarbon ages.
New growth in trees occurs in a layer of cells near the bark. A tree's growth rate changes in a predictable pattern throughout the year in response to seasonal climate changes, resulting in visible growth rings. Each ring marks a complete cycle of seasons, or one year, in the tree's life. As of 2023, securely dated tree-ring data for Germany, Bohemia and Ireland are available going back 13,910 years. A new method is based on measuring variations in oxygen isotopes in each ring, and this 'isotope dendrochronology' can yield results on samples which are not suitable for traditional dendrochronology due to too few or too similar rings. Some regions have "floating sequences", with gaps which mean that earlier periods can only be approximately dated. As of 2024, only three areas have continuous sequences going back to prehistoric times: the foothills of the Northern Alps, the southwestern United States, and the British Isles. Miyake events, which are major spikes in cosmic rays at known dates, are visible in trees rings and can fix the dating of a floating sequence.
== History ==
The Greek botanist Theophrastus (c. 371 – c. 287 BC) first mentioned that the wood of trees has rings. In his 1651 Trattato della Pittura (Treatise on Painting), Leonardo da Vinci (1452–1519) was the first person to mention that trees form rings annually and that their thickness is determined by the conditions under which they grew. In 1737, French investigators Henri-Louis Duhamel du Monceau and Georges-Louis Leclerc de Buffon examined the effect of growing conditions on the shape of tree rings. They found that in 1709, a severe winter produced a distinctly dark tree ring, which served as a reference for subsequent European naturalists. In the U.S., Alexander Catlin Twining (1801–1884) suggested in 1833 that patterns among tree rings could be used to synchronize the dendrochronology of various trees and thereby to reconstruct past climates across entire regions. The English polymath Charles Babbage proposed using dendrochronology to date the remains of trees in peat bogs or even in geological strata (1835, 1838).
During the later half of the nineteenth century, the scientific study of tree rings and the application of dendrochronology began. In 1859, the German-American Jacob Kuechler (1823–1893) used crossdating to examine oaks (Quercus stellata) in order to study the record of climate in western Texas. In 1866, the German botanist, entomologist, and forester Julius Theodor Christian Ratzeburg (1801–1871) observed the effects on tree rings of defoliation caused by insect infestations. By 1882, this observation was already appearing in forestry textbooks. In the 1870s, the Dutch astronomer Jacobus Kapteyn (1851–1922) was using crossdating to reconstruct the climates of the Netherlands and Germany. In 1881, the Swiss-Austrian forester Arthur von Seckendorff-Gudent (1845–1886) was using crossdating. From 1869 to 1901, Robert Hartig (1839–1901), a German professor of forest pathology, wrote a series of papers on the anatomy and ecology of tree rings. In 1892, the Russian physicist Fedor Nikiforovich Shvedov (1841–1905) wrote that he had used patterns found in tree rings to predict droughts in 1882 and 1891.
During the first half of the twentieth century, the astronomer A. E. Douglass founded the Laboratory of Tree-Ring Research at the University of Arizona. Douglass sought to better understand cycles of sunspot activity and reasoned that changes in solar activity would affect climate patterns on earth, which would subsequently be recorded by tree-ring growth patterns (i.e., sunspots → climate → tree rings).
== Methods ==
=== Growth rings ===
Horizontal cross sections cut through the trunk of a tree can reveal growth rings, also referred to as tree rings or annual rings. Growth rings result from new growth in the vascular cambium, a layer of cells near the bark that botanists classify as a lateral meristem; this growth in diameter is known as secondary growth. Visible rings result from the change in growth speed through the seasons of the year; thus, critical for the title method, one ring generally marks the passage of one year in the life of the tree. Removal of the bark of the tree in a particular area may cause deformation of the rings as the plant overgrows the scar.
The rings are more visible in trees which have grown in temperate zones, where the seasons differ more markedly. The inner portion of a growth ring forms early in the growing season, when growth is comparatively rapid (hence the wood is less dense) and is known as "early wood" (or "spring wood", or "late-spring wood"); the outer portion is the "late wood" (sometimes termed "summer wood", often being produced in the summer, though sometimes in the autumn) and is denser.

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Many trees in temperate zones produce one growth-ring each year, with the newest adjacent to the bark. Hence, for the entire period of a tree's life, a year-by-year record or ring pattern builds up that reflects the age of the tree and the climatic conditions in which the tree grew. Adequate moisture and a long growing season result in a wide ring, while a drought year may result in a very narrow one.
Direct reading of tree ring chronologies is a complex science, for several reasons. First, contrary to the single-ring-per-year paradigm, alternating poor and favorable conditions, such as mid-summer droughts, can result in several rings forming in a given year. In addition, particular tree species may present "missing rings", and this influences the selection of trees for study of long time-spans. For instance, missing rings are rare in oak and elm trees.
Critical to the science, trees from the same region tend to develop the same patterns of ring widths for a given period of chronological study. Researchers can compare and match these patterns ring-for-ring with patterns from trees which have grown at the same time in the same geographical zone (and therefore under similar climatic conditions). When one can match these tree-ring patterns across successive trees in the same locale, in overlapping fashion, chronologies can be built up—both for entire geographical regions and for sub-regions. Moreover, wood from ancient structures with known chronologies can be matched to the tree-ring data (a technique called 'cross-dating'), and the age of the wood can thereby be determined precisely. Dendrochronologists originally carried out cross-dating by visual inspection; more recently, they have harnessed computers to do the task, applying statistical techniques to assess the matching. To eliminate individual variations in tree-ring growth, dendrochronologists take the smoothed average of the tree-ring widths of multiple tree-samples to build up a 'ring history', a process termed replication. A tree-ring history whose beginning- and end-dates are not known is called a 'floating chronology'. It can be anchored by cross-matching a section against another chronology (tree-ring history) whose dates are known.
A fully anchored and cross-matched chronology for oak and pine in central Europe extends back 12,460 years, and an oak chronology goes back 7,506 years in Bohemia, 7,429 years in Ireland and 6,939 years in England. Comparison of radiocarbon and dendrochronological ages supports the consistency of these two independent dendrochronological sequences. Another fully anchored chronology that extends back 8,500 years exists for the bristlecone pine in the Southwest US (White Mountains of California).
=== Dendrochronological equation ===
The dendrochronological equation defines the law of growth of tree rings. The equation was proposed by Russian biophysicist Alexandr N. Tetearing in his work "Theory of populations" in the form:
Δ
L
(
t
)
=
1
k
v
ρ
1
3
d
(
M
1
3
(
t
)
)
d
t
,
{\displaystyle \Delta L(t)={\frac {1}{k_{v}\,\rho ^{\frac {1}{3}}}}\,{\frac {d\left(M^{\frac {1}{3}}(t)\right)}{dt}},}
where ΔL is width of annual ring, t is time (in years), ρ is density of wood, kv is some coefficient, M(t) is function of mass growth of the tree.
Ignoring the natural sinusoidal oscillations in tree mass, the formula for the changes in the annual ring width is:
Δ
L
(
t
)
=
−
c
1
e
−
a
1
t
+
c
2
e
−
a
2
t
3
k
v
ρ
1
3
(
c
4
+
c
1
e
−
a
1
t
+
c
2
e
−
a
2
t
)
2
3
{\displaystyle \Delta L(t)=-{\frac {c_{1}e^{-a_{1}t}+c_{2}e^{-a_{2}t}}{3k_{v}\rho ^{\frac {1}{3}}\left(c_{4}+c_{1}e^{-a_{1}t}+c_{2}e^{-a_{2}t}\right)^{\frac {2}{3}}}}}
where c1, c2, and c4 are some coefficients, a1 and a2 are positive constants.
The formula is useful for correct approximation of samples data before data normalization procedure. The typical forms of the function ΔL(t) of annual growth of wood ring are shown in the figures.
=== Sampling and dating ===
Dendrochronology allows specimens of once-living material to be accurately dated to a specific year. Dates are often represented as estimated calendar years B.P., for before present, where "present" refers to 1 January 1950.
Timber core samples are sampled (often using an increment borer) and used to measure the width of annual growth rings; by taking samples from different sites within a particular region, researchers can build a comprehensive historical sequence. The techniques of dendrochronology are more consistent in areas where trees grew in marginal conditions such as aridity or semi-aridity where the ring growth is more sensitive to the environment, rather than in humid areas where tree-ring growth is more uniform (complacent). In addition, some genera of trees are more suitable than others for this type of analysis. For instance, the bristlecone pine is exceptionally long-lived and slow growing, and has been used extensively for chronologies; still-living and dead specimens of this species provide tree-ring patterns going back thousands of years, in some regions more than 10,000 years. Currently, the maximum span for fully anchored chronology is a little over 11,000 years B.P.
IntCal20 is the 2020 "Radiocarbon Age Calibration Curve", which provides a calibrated carbon 14 dated sequence going back 55,000 years. The most recent part, going back 13,900 years, is based on tree rings.
=== Reference sequences ===
European chronologies derived from wooden structures initially found it difficult to bridge the gap in the fourteenth century when there was a building hiatus, which coincided with the Black Death. However, there do exist unbroken chronologies dating back to prehistoric times, for example the Danish chronology dating back to 352 BC.
Given a sample of wood, the variation of the tree-ring growths not only provides a match by year, but can also match location because climate varies from place to place. This makes it possible to determine the source of ships as well as smaller artifacts made from wood, but which were transported long distances, such as panels for paintings and ship timbers.

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=== Miyake events ===
Miyake events, such as the ones in 774–775 and 993–994, can provide fixed reference points in an unknown time sequence as they are due to cosmic radiation. As they appear as spikes in carbon 14 in tree rings for that year all round the world, they can be used to date historical events to the year. For example, wooden houses in the Viking site at L'Anse aux Meadows in Newfoundland were dated by finding the layer with the 993 spike, which showed that the wood is from a tree felled in 1021. Researchers at the University of Bern have provided exact dating of a floating sequence in a Neolithic settlement in northern Greece by tying it to a spike in cosmogenic radiocarbon in 5259 BC.
=== Frost rings ===
Frost ring is a term used to designate a layer of deformed, collapsed tracheids and traumatic parenchyma cells in tree ring analysis. They are formed when air temperature falls below freezing during a period of cambial activity. They can be used in dendrochronology to indicate years that are colder than usual.
== Applications ==
=== Radiocarbon dating calibration ===
Dates from dendrochronology can be used as a calibration and check of radiocarbon dating. This can be done by checking radiocarbon dates against long master sequences, with Californian bristle-cone pines in Arizona being used to develop this method of calibration as the longevity of the trees (up to c.4900 years) in addition to the use of dead samples meant a long, unbroken tree ring sequence could be developed (dating back to c. 6700 BC). Additional studies of European oak trees, such as the master sequence in Germany that dates back to c. 8500 BC, can also be used to back up and further calibrate radiocarbon dates.
=== Climatology ===
Dendroclimatology is the science of determining past climates from trees primarily from the properties of the annual tree rings. Other properties of the annual rings, such as maximum latewood density (MXD) have been shown to be better proxies than simple ring width. Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous.
=== Art history ===
Dendrochronology has become important to art historians in the dating of panel paintings. However, unlike analysis of samples from buildings, which are typically sent to a laboratory, wooden supports for paintings usually have to be measured in a museum conservation department, which places limitations on the techniques that can be used.
In addition to dating, dendrochronology can also provide information as to the source of the panel. Many Early Netherlandish paintings have turned out to be painted on panels of "Baltic oak" shipped from the Vistula region via ports of the Hanseatic League. Oak panels were used in a number of northern countries such as England, France and Germany. Wooden supports other than oak were rarely used by Netherlandish painters.
Since panels of seasoned wood were used, an uncertain number of years has to be allowed for seasoning when estimating dates. Panels were trimmed of the outer rings, and often each panel only uses a small part of the radius of the trunk. Consequently, dating studies usually result in a terminus post quem (earliest possible) date, and a tentative date for the arrival of a seasoned raw panel using assumptions as to these factors. As a result of establishing numerous sequences, it was possible to date 85–90% of the 250 paintings from the fourteenth to seventeenth century analysed between 1971 and 1982; by now a much greater number have been analysed.
A portrait of Mary, Queen of Scots in the National Portrait Gallery, London was believed to be an eighteenth-century copy. However, dendrochronology revealed that the wood dated from the second half of the sixteenth century. It is now regarded as an original sixteenth-century painting by an unknown artist.
On the other hand, dendrochronology was applied to four paintings depicting the same subject, that of Christ expelling the money-lenders from the Temple. The results showed that the age of the wood was too late for any of them to have been painted by Hieronymus Bosch.
While dendrochronology has become an important tool for dating oak panels, it is not effective in dating the poplar panels often used by Italian painters because of the erratic growth rings in poplar.
The sixteenth century saw a gradual replacement of wooden panels by canvas as the support for paintings, which means the technique is less often applicable to later paintings. In addition, many panel paintings were transferred onto canvas or other supports during the nineteenth and twentieth centuries.
=== Archaeology ===
The dating of buildings with wooden structures and components is also done by dendrochronology; dendroarchaeology is the term for the application of dendrochronology in archaeology. While archaeologists can date wood and when it was felled, it may be difficult to definitively determine the age of a building or structure in which the wood was used; the wood could have been reused from an older structure, may have been felled and left for many years before use, or could have been used to replace a damaged piece of wood. The dating of building via dendrochronology thus requires knowledge of the history of building technology. Many prehistoric forms of buildings used "posts" that were whole young tree trunks; where the bottom of the post has survived in the ground these can be especially useful for dating.
Examples:

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The Post Track and Sweet Track, ancient timber trackways in the Somerset levels, England, have been dated to 3838 BC and 3807 BC.
Navan Fort where in Prehistoric Ireland a large structure was built with more than two hundred posts. The central oak post was felled in 95 BC.
The Fairbanks House in Dedham, Massachusetts. While the house had long been claimed to have been built c. 1640 (and being the oldest wood-framed house in North America), core samples of wood taken from a summer beam confirmed the wood was from an oak tree felled in 1637–8, as wood was not seasoned before use in building at that time in New England. An additional sample from another beam yielded a date of 1641, thus confirming the house had been constructed starting in 1638 and finished sometime after 1641 .
The burial chamber of Gorm the Old, who died c. 958, was constructed from wood of timbers felled in 958.
Veliky Novgorod, where, between the tenth and the fifteenth century, numerous consecutive layers of wooden log pavement have been placed over the accumulating dirt.
The Neolithic well with linings made of oak wood, found near Ostrov, Czech Republic, have been dated to 5,482-5,243 BC.
== Measurement platforms, software, and data formats ==
There are many different file formats used to store tree ring width data. Effort for standardisation was made with the development of TRiDaS. Further development led to the database software Tellervo, which is based on the new standard format whilst being able to import lots of different data formats. The desktop application can be attached to measurement devices and works with the database server that is installed separately.
== Continuous sequence ==
Bard et al write in 2023: "The oldest tree-ring series are known as floating since, while their constituent rings can be counted to create a relative internal chronology, they cannot be dendro-matched with the main Holocene absolute chronology. However, 14C analyses performed at high resolution on overlapped absolute and floating tree-rings series enable one to link them almost absolutely and hence to extend the calibration on annual tree rings until ≈13 900 cal yr BP."
Some of the longest tree-ring timelines, especially those extending earlier than about 4000 BC, are joined by comparing how similar the ring patterns look instead of by directly overlapping pieces of wood from different trees. In those sections only a few samples match, so the connection is less certain.
== Related chronologies ==
Herbchronology is the analysis of annual growth rings (or simply annual rings) in the secondary root xylem of perennial herbaceous plants. Similar seasonal patterns also occur in ice cores and in varves (layers of sediment deposition in a lake, river, or sea bed). The deposition pattern in the core will vary for a frozen-over lake versus an ice-free lake, and with the fineness of the sediment. Sclerochronology is the study of algae deposits.
Some columnar cacti also exhibit similar seasonal patterns in the isotopes of carbon and oxygen in their spines (acanthochronology). These are used for dating in a manner similar to dendrochronology, and such techniques are used in combination with dendrochronology, to plug gaps and to extend the range of the seasonal data available to archaeologists and paleoclimatologists.
A similar technique is used to estimate the age of fish stocks through the analysis of growth rings in the otolith bones.
== See also ==
Dendrology
International Tree-Ring Data Bank
Post excavation
Timeline of dendrochronology timestamp events
== References ==
== External links ==
Nottingham Tree-Ring Dating Laboratory
Oxford Tree-Ring Laboratory
Dendrochronology and Art History of Painted Ceilings (Historic Environment Scotland, 2017).
Video & commentary on medullary rays, heart wood, and tree rings.
Video & commentary on Tree Rings – Formation and Purpose
Bibliography of Dendrochronology
Multilingual Glossary of Dendrochronology
Digital Collaboratory for Cultural Dendrochronology (DCCD)
International Tree-Ring Data Bank
Laboratory of Tree-Ring Research University of Arizona
"Tree Ring Science", the academic site of Prof. Henri D. Grissino-Mayer, Department of Geography, The University of Tennessee, and the Laboratory of Tree-Ring Science
Briand, Christopher H.; Brazer, Susan E.; Harter-Dennis, Jeannine M. (December 2006). "Tree Rings and the Aging of Trees: A Controversy in 19th Century America". Tree-Ring Research. 62 (2): 51–65. doi:10.3959/1536-1098-62.2.51. hdl:10150/262645. S2CID 162884050.

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Empirical evidence is evidence obtained through sense experience or experimental procedure. It is of central importance to the sciences and plays a role in various other fields, like epistemology and law.
There is no general agreement on how the terms evidence and empirical are to be defined. Often different fields work with quite different conceptions. In epistemology, evidence is what justifies beliefs or what determines whether holding a certain belief is rational. This is only possible if the evidence is possessed by the person, which has prompted various epistemologists to conceive evidence as private mental states like experiences or other beliefs. In philosophy of science, on the other hand, evidence is understood as that which confirms or disconfirms scientific hypotheses and arbitrates between competing theories. For this role, evidence must be public and uncontroversial, like observable physical objects or events and unlike private mental states, so that evidence may foster scientific consensus. The term empirical comes from Greek ἐμπειρία empeiría, i.e. 'experience'. In this context, it is usually understood as what is observable, in contrast to unobservable or theoretical objects. It is generally accepted that unaided perception constitutes observation, but it is disputed to what extent objects accessible only to aided perception, like bacteria seen through a microscope or positrons detected in a cloud chamber, should be regarded as observable.
Empirical evidence is essential to a posteriori knowledge or empirical knowledge, knowledge whose justification or falsification depends on experience or experiment. A priori knowledge, on the other hand, is seen either as innate or as justified by rational intuition and therefore as not dependent on empirical evidence. Rationalism fully accepts that there is knowledge a priori, which is either outright rejected by empiricism or accepted only in a restricted way as knowledge of relations between our concepts but not as pertaining to the external world.
Scientific evidence is closely related to empirical evidence but not all forms of empirical evidence meet the standards dictated by scientific methods. Sources of empirical evidence are sometimes divided into observation and experimentation, the difference being that only experimentation involves manipulation or intervention: phenomena are actively created instead of being passively observed.
== Background ==
The concept of evidence is of central importance in epistemology and in philosophy of science but plays different roles in these two fields. In epistemology, evidence is what justifies beliefs or what determines whether holding a certain doxastic attitude is rational. For example, the olfactory experience of smelling smoke justifies or makes it rational to hold the belief that something is burning. It is usually held that for justification to work, the evidence has to be possessed by the believer. The most straightforward way to account for this type of evidence possession is to hold that evidence consists of the private mental states possessed by the believer.
Some philosophers restrict evidence even further, for example, to only conscious, propositional or factive mental states. Restricting evidence to conscious mental states has the implausible consequence that many simple everyday beliefs would be unjustified. This is why it is more common to hold that all kinds of mental states, including stored but currently unconscious beliefs, can act as evidence. Various of the roles played by evidence in reasoning, for example, in explanatory, probabilistic and deductive reasoning, suggest that evidence has to be propositional in nature, i.e. that it is correctly expressed by propositional attitude verbs like "believe" together with a that-clause, like "that something is burning". But it runs counter to the common practice of treating non-propositional sense-experiences, like bodily pains, as evidence. Its defenders sometimes combine it with the view that evidence has to be factive, i.e. that only attitudes towards true propositions constitute evidence. In this view, there is no misleading evidence. The olfactory experience of smoke would count as evidence if it was produced by a fire but not if it was produced by a smoke generator. This position has problems in explaining why it is still rational for the subject to believe that there is a fire even though the olfactory experience cannot be considered evidence.
In philosophy of science, evidence is understood as that which confirms or disconfirms scientific hypotheses and arbitrates between competing theories. Measurements of Mercury's "anomalous" orbit, for example, constitute evidence that plays the role of neutral arbiter between Newton's and Einstein's theory of gravitation by confirming Einstein's theory. For scientific consensus, it is central that evidence is public and uncontroversial, like observable physical objects or events and unlike private mental states. This way it can act as a shared ground for proponents of competing theories. Two issues threatening this role are the problem of underdetermination and theory-ladenness. The problem of underdetermination concerns the fact that the available evidence often provides equal support to either theory and therefore cannot arbitrate between them. Theory-ladenness refers to the idea that evidence already includes theoretical assumptions. These assumptions can hinder it from acting as neutral arbiter. It can also lead to a lack of shared evidence if different scientists do not share these assumptions. Thomas Kuhn is an important advocate of the position that theory-ladenness concerning scientific paradigms plays a central role in science.

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== Definition ==
A thing is evidence for a proposition if it epistemically supports this proposition or indicates that the supported proposition is true. Evidence is empirical if it is constituted by or accessible to sensory experience. There are various competing theories about the exact definition of the terms evidence and empirical. Different fields, like epistemology, the sciences or legal systems, often associate different concepts with these terms. An important distinction among theories of evidence is whether they identify evidence with private mental states or with public physical objects. Concerning the term empirical, there is a dispute about where to draw the line between observable or empirical objects in contrast to unobservable or merely theoretical objects.
The traditional view proposes that evidence is empirical if it is constituted by or accessible to sensory experience. This involves experiences arising from the stimulation of the sense organs, like visual or auditory experiences, but the term is often used in a wider sense including memories and introspection. It is usually seen as excluding purely intellectual experiences, like rational insights or intuitions used to justify basic logical or mathematical principles. The terms empirical and observable are closely related and sometimes used as synonyms.
There is an active debate in contemporary philosophy of science as to what should be regarded as observable or empirical in contrast to unobservable or merely theoretical objects. There is general consensus that everyday objects like books or houses are observable since they are accessible via unaided perception, but disagreement starts for objects that are only accessible through aided perception. This includes using telescopes to study distant galaxies, microscopes to study bacteria or using cloud chambers to study positrons. So the question is whether distant galaxies, bacteria or positrons should be regarded as observable or merely theoretical objects. Some even hold that any measurement process of an entity should be considered an observation of this entity. In this sense, the interior of the Sun is observable since neutrinos originating there can be detected. The difficulty with this debate is that there is a continuity of cases going from looking at something with the naked eye, through a window, through a pair of glasses, through a microscope, etc. Because of this continuity, drawing the line between any two adjacent cases seems to be arbitrary. One way to avoid these difficulties is to hold that it is a mistake to identify the empirical with what is observable or sensible. Instead, it has been suggested that empirical evidence can include unobservable entities as long as they are detectable through suitable measurements. A problem with this approach is that it is rather far from the original meaning of "empirical", which contains the reference to experience.
== Related concepts ==
=== Knowledge a posteriori and a priori ===
Knowledge or the justification of a belief is said to be a posteriori if it is based on empirical evidence. A posteriori refers to what depends on experience (what comes after experience), in contrast to a priori, which stands for what is independent of experience (what comes before experience). For example, the proposition that "all bachelors are unmarried" is knowable a priori since its truth only depends on the meanings of the words used in the expression. The proposition "some bachelors are happy", on the other hand, is only knowable a posteriori since it depends on experience of the world as its justifier. Immanuel Kant held that the difference between a posteriori and a priori is tantamount to the distinction between empirical and non-empirical knowledge.
Two central questions for this distinction concern the relevant sense of "experience" and of "dependence". The paradigmatic justification of knowledge a posteriori consists in sensory experience, but other mental phenomena, like memory or introspection, are also usually included in it. But purely intellectual experiences, like rational insights or intuitions used to justify basic logical or mathematical principles, are normally excluded from it. There are different senses in which knowledge may be said to depend on experience. In order to know a proposition, the subject has to be able to entertain this proposition, i.e. possess the relevant concepts. For example, experience is necessary to entertain the proposition "if something is red all over then it is not green all over" because the terms "red" and "green" have to be acquired this way. But the sense of dependence most relevant to empirical evidence concerns the status of justification of a belief. So experience may be needed to acquire the relevant concepts in the example above, but once these concepts are possessed, no further experience providing empirical evidence is needed to know that the proposition is true, which is why it is considered to be justified a priori.

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=== Empiricism and rationalism ===
In its strictest sense, empiricism is the view that all knowledge is based on experience or that all epistemic justification arises from empirical evidence. This stands in contrast to the rationalist view, which holds that some knowledge is independent of experience, either because it is innate or because it is justified by reason or rational reflection alone. Expressed through the distinction between knowledge a priori and a posteriori from the previous section, rationalism affirms that there is knowledge a priori, which is denied by empiricism in this strict form. One difficulty for empiricists is to account for the justification of knowledge pertaining to fields like mathematics and logic, for example, that 3 is a prime number or that modus ponens is a valid form of deduction. The difficulty is due to the fact that there seems to be no good candidate of empirical evidence that could justify these beliefs. Such cases have prompted empiricists to allow for certain forms of knowledge a priori, for example, concerning tautologies or relations between our concepts. These concessions preserve the spirit of empiricism insofar as the restriction to experience still applies to knowledge about the external world. In some fields, like metaphysics or ethics, the choice between empiricism and rationalism makes a difference not just for how a given claim is justified but for whether it is justified at all. This is best exemplified in metaphysics, where empiricists tend to take a skeptical position, thereby denying the existence of metaphysical knowledge, while rationalists seek justification for metaphysical claims in metaphysical intuitions.
=== Scientific evidence ===
Scientific evidence is closely related to empirical evidence. Some theorists, like Carlos Santana, have argued that there is a sense in which not all empirical evidence constitutes scientific evidence. One reason for this is that the standards or criteria that scientists apply to evidence exclude certain evidence that is legitimate in other contexts. For example, anecdotal evidence from a friend about how to treat a certain disease constitutes empirical evidence that this treatment works but would not be considered scientific evidence. Others have argued that the traditional empiricist definition of empirical evidence as perceptual evidence is too narrow for much of scientific practice, which uses evidence from various kinds of non-perceptual equipment.
Central to scientific evidence is that it was arrived at by following scientific method in the context of some scientific theory. But people rely on various forms of empirical evidence in their everyday lives that have not been obtained this way and therefore do not qualify as scientific evidence. One problem with non-scientific evidence is that it is less reliable, for example, due to cognitive biases like the anchoring effect, in which information obtained earlier is given more weight, although science done poorly is also subject to such biases, as in the example of p-hacking.
=== Observation, experimentation and scientific method ===
In the philosophy of science, it is sometimes held that there are two sources of empirical evidence: observation and experimentation. The idea behind this distinction is that only experimentation involves manipulation or intervention: phenomena are actively created instead of being passively observed. For example, inserting viral DNA into a bacterium is a form of experimentation while studying planetary orbits through a telescope belongs to mere observation. In these cases, the mutated DNA was actively produced by the biologist while the planetary orbits are independent of the astronomer observing them. Applied to the history of science, it is sometimes held that ancient science is mainly observational while the emphasis on experimentation is only present in modern science and responsible for the Scientific Revolution. This is sometimes phrased through the expression that modern science actively "puts questions to nature". This distinction also underlies the categorization of sciences into experimental sciences, like physics, and observational sciences, like astronomy. While the distinction is relatively intuitive in paradigmatic cases, it has proven difficult to give a general definition of "intervention" applying to all cases, which is why it is sometimes outright rejected.
Empirical evidence is required for a hypothesis to gain acceptance in the scientific community. Normally, this validation is achieved by the scientific method of forming a hypothesis, experimental design, peer review, reproduction of results, conference presentation, and journal publication. This requires rigorous communication of hypothesis (usually expressed in mathematics), experimental constraints and controls (expressed in terms of standard experimental apparatus), and a common understanding of measurement. In the scientific context, the term semi-empirical is used for qualifying theoretical methods that use, in part, basic axioms or postulated scientific laws and experimental results. Such methods are opposed to theoretical ab initio methods, which are purely deductive and based on first principles. Typical examples of both ab initio and semi-empirical methods can be found in computational chemistry.
== See also ==
== Footnotes ==
== References ==
Bird, Alexander (2013). "Thomas Kuhn". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy. Section 4.2 Perception, Observational Incommensurability, and World-Change. Retrieved 25 January 2012.
Craig, Edward (2005). "a posteriori". The Shorter Routledge Encyclopedia of Philosophy. Routledge. ISBN 978-0415324953.
Feldman, Richard (2001) [1999]. "Evidence". In Audi, Robert (ed.). The Cambridge Dictionary of Philosophy (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 293–294. ISBN 978-0521637220.
Kuhn, Thomas S. (1970) [1962]. The Structure of Scientific Revolutions (2nd ed.). Chicago: University of Chicago Press. ISBN 978-0226458045.
Pickett, Joseph P., ed. (2011). The American Heritage Dictionary of the English Language (5th ed.). Houghton Mifflin. ISBN 978-0-547-04101-8.
== External links ==
The dictionary definition of empirical at Wiktionary
The dictionary definition of evidence at Wiktionary
Fieser, James; Dowden, Bradley (eds.). "A Priori and A Posteriori". Internet Encyclopedia of Philosophy. ISSN 2161-0002. OCLC 37741658.

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The roughly spherical shape of Earth can be empirically evidenced by many different types of observation, ranging from ground level, flight, or orbit. The spherical shape causes a number of effects and phenomena that when combined disprove flat Earth beliefs.
These include the visibility of distant objects on Earth's surface; lunar eclipses; appearance of the Moon; observation of the sky from a certain altitude; observation of certain fixed stars from different locations; observing the Sun; surface navigation; grid distortion on a spherical surface; weather systems; gravity; and modern technology.
== Visibility of distant objects on Earth's surface ==
On a completely flat Earth without obstructions (mountains, hills, valleys or volcanos), the ground itself would never obscure distant objects. A spherical surface has a horizon which is closer when viewed from a lower altitude. In theory, a person standing on the surface with eyes 1.8 metres (5 ft 11 in) above the ground can see the ground up to about 4.79 kilometres (2.98 mi) away, but a person at the top of the Eiffel Tower at 273 metres (896 ft) can see the ground up to about 58.98 kilometres (36.65 mi) away.
This phenomenon permits a way of confirming that Earth's surface is locally convex: If the degree of curvature is determined to be the same everywhere on Earth's surface, and that surface was determined to be large enough, the constant curvature would show that Earth is spherical. In practice, this method is not reliable because of variations in atmospheric refraction, which is how much the atmosphere bends light traveling through it. Refraction can give the impression that Earth's surface is flat, curved more convexly than it is, or even that it is concave (this is what happened in various trials of the Bedford Level experiment).
The phenomenon of variable atmospheric bending can be seen when distant objects appear to be broken into pieces or even turned upside down. This is often seen at sunset, when the Sun's shape is distorted, but has also been photographed happening to ships, and has caused the city of Chicago to appear normally, upside down, and broken into pieces from across Lake Michigan (from where it is normally below the horizon).
When the atmosphere is relatively well-mixed, the visual effects generally expected of a spherical Earth can be observed. For example, ships travelling on large bodies of water (such as the ocean) disappear over the horizon progressively, such that the highest part of the ship can still be seen even when lower parts cannot, proportional to distance from the observer. Likewise, in the days of sailing ships, a sailor would climb up a mast to see farther. The same is true of the coastline or mountain when viewed from a ship or from across a large lake or flat terrain. In certain places, the curvature is visible via fixed objects. This includes the 23-mile (37 km) Lake Pontchartrain Causeway visible from a Metairie hotel, and the 85 pylons carrying 15 miles (24 km) of powerlines over Lake Pontchartrain, visible from I-10 Bonnet Carré Spillway Bridge.
== Lunar eclipses ==
The shadow of Earth on the Moon during a lunar eclipse is always a dark circle that moves from one side of the Moon to the other (partially grazing it during a partial eclipse). The only shape that casts a round shadow no matter which direction it is pointed is a sphere, and the ancient Greeks deduced that this must mean Earth is spherical.
The effect could be produced by a disk that always faces the Moon head-on during the eclipse, but this is inconsistent with the fact that the Moon is only rarely directly overhead during an eclipse. For each eclipse, the local surface of Earth is pointed in a different direction. The shadow of a disk held at an angle is an oval, not a circle as is seen during the eclipse. The idea of Earth being a disk is also inconsistent with the fact that a given lunar eclipse is only visible from half of Earth at a time.
== Appearance of the Moon ==
The Moon's tidal lock to Earth results in the Moon's always showing only one side to Earth (see animated image). If Earth were flat, with the Moon hovering above it, then the portion of the Moon's surface visible to people on Earth would vary according to location on Earth, rather than showing an identical "face side" to everyone. If Earth were flat, with the Moon revolving around it tidally locked, then the Moon would be seen simultaneously at all places on Earth at once, but its apparent size, the portion facing the viewer, and facing side's orientation would gradually change for each viewer as its position moved across the sky over the course of the night.

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== Observation of the sky from altitude with the aid of a diagram ==
On a perfectly spherical Earth, not considering obstructions and atmospheric refraction, its surface blocks almost half the sky for an observer close against the surface (see horizon). Moving away from the surface of Earth means that the ground blocks less and less of the sky. For example, when viewed from the Moon, Earth blocks only a small portion of the sky because it is so distant. This effect of geometry means that, when viewed from a high mountain, flat ground or ocean blocks less than a hemisphere of the sky. With the presumption of a spherical Earth, an expedition commissioned by caliph al-Ma'mun used this fact to calculate Earth's circumference to within 7,920 kilometres (4,920 mi) of the correct value of around 40,000 kilometres (25,000 mi), and possibly as accurately as 180 kilometres (110 mi).
The rate of change in the angle blocked by Earth as altitude increases would be different for a disk than for a sphere. The amount of surface blocked would be different for a mountain close to the edge of a flat Earth compared to a mountain in the middle of a flat Earth, but this is not observed. Surveys from all over Earth show that its shape is everywhere locally convex, confirming that it is very close to spherical.
== Observation of fixed stars from different locations ==
The fixed stars, for example the Pole Star (Polaris), can be demonstrated to be very far away by diurnal parallax measurements. Such measurements show no shifts in the stars' positions. Unlike the Sun, Moon, and planets, they do not change position with respect to one another over human lifetimes; the shapes of the constellations are constant. This makes them a convenient reference background for determining the shape of Earth. Adding distance measurements on the ground allows calculation of Earth's size.
The fact that different stars are visible from different locations on Earth was noticed in ancient times. Aristotle wrote that some stars are visible from Egypt which are not visible from Europe. This would not be possible if Earth was flat.
A star has an altitude above the horizon for an observer if the star is visible. Observing the same star at the same time from two different latitudes gives two different altitudes. Using geometry, the two altitudes along with the distance between the two locations allows for a calculation of Earth's size. Using observations of the star Canopus at Rhodes (in Greece) and Alexandria (in Egypt) and the distance between them, the Ancient Greek philosopher Posidonius used this technique to calculate the circumference of the planet to within perhaps 4% of the correct value. Modern equivalents of his units of measure are not precisely known, so it is not clear how accurate his measurement was.
The Andalusian astronomer Ibn Rushd went to Marrakesh (in Morocco) to observe the same star in 1153, as it was invisible in his native Córdoba, Al-Andalus. He used the different visibility in different latitudes to argue that the Earth is round, following Aristotle's argument.
=== Observation of constellations on North and South hemispheres at different seasons ===
The North Pole is in continuous night for six months of the year. The star Polaris (the "North Star") is almost directly overhead and therefore at the center of this rotation. Some of the 88 modern constellations visible are Ursa Major (including the Big Dipper), Cassiopeia, and Andromeda. The other six months of the year, the North Pole is in continuous daylight, with light from the Sun blotting out the stars. This phenomenon, and its analogous effects at the South Pole, are what defines the two poles. More than 24 hours of continuous daylight can only occur north of the Arctic Circle and south of the Antarctic Circle.)
At the South Pole, a completely different set of constellations are visible during the six months of continuous night, including Crux, and Centaurus. This 180° hemisphere of stars rotates clockwise once every 24 hours around a point directly overhead.
From any point on the equator, all of the stars visible anywhere on Earth on that day are visible at some time during the year as the sky rotates around a line drawn from due north to due south. When facing east, the stars visible from the north pole are on the left, and the stars visible from the south pole are on the right.
The direction any intermediate spot on Earth is facing can also be calculated by measuring the angles of the fixed stars and determining how much of the sky is visible. For example, New York City is about 40° north of the equator. The apparent motion of the Sun blots out slightly different parts of the sky from day to day, but over the course of the entire year it sees a dome of 280° (360° - 80°). So for example, both Orion and the Big Dipper are visible during at least part of the year.
Making stellar observations from a representative set of points across Earth, combined with knowing the shortest on-the-ground distance between any two given points, makes an approximate sphere the only possible shape for Earth.

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== Observing the Sun ==
On a flat Earth, a Sun that shines in all directions would illuminate the entire surface at the same time, and all places would experience sunrise and sunset at the horizon at about the same time. With a spherical Earth, half the planet is in daylight at any given time and the other half experiences nighttime. When a given location on the spherical Earth is in sunlight, its antipode – the location exactly on the opposite side of Earth – is in darkness. The spherical shape of Earth causes the Sun to rise and set at different times in different places, and different locations get different amounts of sunlight each day.
In order to explain day and night, time zones, and the seasons, some flat Earth theorists propose that the Sun does not emit light in all directions, but acts more like a spotlight, only illuminating part of the flat Earth at a time. This conjecture is not consistent with observation: At sunrise and sunset, a spotlight Sun would be up in the sky at least a little bit, rather than at the horizon where it is always actually observed. A spotlight Sun would also appear at different angles in the sky with respect to a flat ground than it does with respect to a curved ground. Assuming light travels in straight lines, actual measurements of the Sun's angle in the sky from locations very distant from each other are only consistent with a geometry where the Sun is very far away and is being seen from the daylight half of a spherical Earth. These two phenomena are related: A low-altitude spotlight Sun would spend most of the day near the horizon for most locations on Earth, which is not observed, but rise and set fairly close to the horizon. A high-altitude Sun would spend more of the day away from the horizon, but rise and set fairly far from the horizon, which is also not observed.
=== Changing length of the day ===
On a flat Earth with an omnidirectional Sun, all places would experience the same amount of daylight every day, and all places would get daylight at the same time. Actual day length varies considerably, with places closer to the poles getting very long days in the summer and very short days in the winter, with northerly summer happening at the same time as southerly winter, and vice versa. Places north of the Arctic Circle and south of the Antarctic Circle get no sunlight for at least one day a year, and get 24-hour sunlight for at least one day a year. Both the poles experience sunlight for 6 months and darkness for 6 months, at opposite times.
The movement of daylight between the northern and southern hemispheres happens because of the axial tilt of Earth. The imaginary line around which Earth spins, which goes between the North Pole and South Pole, is tilted about 23° from the oval that describes its orbit around the Sun. Earth always points in the same direction as it moves around the Sun, so for half the year (summer in the Northern Hemisphere), the North Pole is pointed slightly toward the Sun, keeping it in daylight all the time because the Sun lights up the half of Earth that is facing it (and the North Pole is always in that half due to the tilt). For the other half of the orbit, the South Pole is tilted slightly toward the Sun, and it is winter in the Northern Hemisphere. This means that at the equator, the Sun is not directly overhead at noon, except around the March and September equinoxes, when one spot on the equator is pointed directly at the Sun.
=== Length of the day beyond polar circles ===
The length of the day varies because as Earth rotates, some places (near the poles) pass through only a short curve near the top or bottom of the sunlight half; other places (near the equator) travel along much longer curves through the middle. In locations just outside the polar circles, there are so-called "white nights" in the middle of summer, in which the sun is never more than a few degrees below the horizon in June such that a bright twilight persists from sunset to sunrise. In Russia, Saint Petersburg uses this phenomenon in its tourist marketing.
=== Length of the twilight ===
Longer twilights are observed at higher latitudes (near the poles) due to a shallower angle of the Sun's apparent movement compared to the horizon. On a flat Earth, the Sun's shadow would reach the upper atmosphere very quickly, except near the closest edge of Earth, and would always set at the same angle to the ground (which is not what is observed).
The length of twilight would be very different on a flat Earth. On a round Earth, the atmosphere above the ground is lit for a while before sunrise and after sunset are observed at ground level, because the Sun is still visible from higher altitudes.
The "spotlight Sun" conjecture is also not consistent with this observation, since the air cannot be lit without the ground below it also being lit (except for shadows of mountains, hi-rises and other surface obstacles).

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=== Observing sunlight before or after seeing Sun ===
It is possible to see sun-lit windows of nearby high-rise buildings from ground level a few minutes before seeing the sun rise or after seeing the sun set. On a non-curved, flat landmass it would only take seconds, due to minuscule ratio (compare ~45 meters / 150 feet of a 14-story building to intercontinental distances). If such a phenomenon were caused by a prismatic property of atmosphere in a flat world, with a relatively small source of light revolving around Earth (as in later, 1800's-dated, maps of Flat Earth), it would contradict with one's ability to see a proper panorama of starry sky at a time at night, rather than a small yet distorted, "stretched" patch of it.
Likewise, the top of a mountain is illuminated before sunrise and after sunset, as are clouds.
=== Watching the sun set twice ===
On level ground, the difference in the distance to the horizon between lying down and standing up is large enough to watch the Sun set twice by quickly standing up immediately after seeing it set for the first time while lying down. This also can be done with an aerial work platform or with a fast elevator. On a flat Earth or a significantly large flat segment, it would not be possible to see the Sun again (unless standing near the edge closest to the Sun) due to a much faster-moving Sun shadow.
=== Local solar time and time zones ===
Ancient timekeeping reckoned "noon" as the time of day when the Sun is highest in the sky, with the rest of the hours in the day measured against that. During the day, the apparent solar time can be measured directly with a sundial. In ancient Egypt, the first known sundials divided the day into 12 hours, though because the length of the day changed with the season, the length of the hours also changed. Sundials that defined hours as always being the same duration appeared in the Renaissance. In Western Europe, clock towers and striking clocks were used in the Middle Ages to keep people nearby appraised of the local time, though compared to modern times this was less important in a largely agrarian society.
Because the Sun reaches its highest point at different times for different longitudes (about four minutes of time for every degree of longitude difference east or west), the local solar noon in each city is different except for those directly north or south of each other. This means that the clocks in different cities could be offset from each other by minutes or hours. As clocks became more precise and industrialization made timekeeping more important, cities switched to mean solar time, which ignores minor variations in the timing of local solar noon over the year, due to the elliptical nature of Earth's orbit, and its tilt.
The differences in clock time between cities was not generally a problem until the advent of railroad travel in the 1800s, which both made travel between distant cities much faster than by walking or horse, and also required passengers to show up at specific times to meet their desired trains. In the United Kingdom, railroads gradually switched to Greenwich Mean Time (set from local time at the Greenwich observatory in London), followed by public clocks across the country generally, forming a single time zone. In the United States, railroads published schedules based on local time, then later based on standard time for that railroad (typically the local time at the railroad's headquarters), and then finally based on four standard time zones shared across all railroads, where neighboring zones differed by exactly one hour. At first railroad time was synchronized by portable chronometers, and then later by telegraph and radio signals.
San Francisco is at 122.41°W longitude and Richmond, Virginia, is at 77.46°W longitude. They are both at about 37.6°N latitude (±.2°). The approximately 45° of longitude difference translates into about 180 minutes, or 3 hours, of time between sunsets in the two cities, for example. San Francisco is in the Pacific Time zone, and Richmond is in the Eastern Time zone, which are three hours apart, so the local clocks in each city show that the Sun sets at about the same time when using the local time zone. But a phone call from Richmond to San Francisco at sunset will reveal that there are still three hours of daylight left in California.
=== Determining the size of Earth by Eratosthenes ===
Under the assumption that the Sun is very far away, the ancient Greek geographer Eratosthenes performed an experiment using the differences in the observed angle of the Sun from two different locations to calculate the circumference of Earth. Though modern telecommunications and timekeeping were not available, he was able to make sure the measurements happened at the same time by having them taken when the Sun was highest in the sky (local noon) at both locations. Using slightly inaccurate assumptions about the locations of two cities, he came to a result within 15% of the correct value. While his results could theoretically also be compatible with a Flat Earth if the light rays from the Sun are assumed not to be parallel, many people have repeated the experiment with three or more data points and found results unambiguously supporting the globe model.

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=== Angle to the Sun at different locations ===
On a given day, if many different cities measure the angle of the Sun at local noon, the resulting data, when combined with the known distances between cities, shows that Earth has 180 degrees of north-south curvature. (A full range of angles will be observed if the north and south poles are included, and the day chosen is either the autumnal or spring equinox.) This is consistent with many rounded shapes, including a sphere, and is inconsistent with a flat shape.
Some claim that this experiment assumes a very distant Sun, such that the incoming rays are essentially parallel, and if a flat Earth is assumed, that the measured angles can allow one to calculate the distance to the Sun, which must be small enough that its incoming rays are not very parallel. However, if more than two relatively well-separated cities are included in the experiment, the calculation will make clear whether the Sun is distant or nearby. For example, on the equinox, the 0-degree angle from the North Pole and the 90-degree angle from the equator predict a Sun which would have to be located essentially next to the surface of a flat Earth, but the difference in angle between the equator and New York City would predict a Sun much further away if Earth is flat. Because these results are contradictory, the surface of Earth cannot be flat; the data are, instead, consistent with a nearly spherical Earth and a Sun which is very far away compared with the diameter of Earth.
== Surface navigation ==
The first circumnavigation of the Earth by the Magellan expedition lost a day, confirmed by subsequent circumnavigations, which eventually led to the creation of the International Date Line.
The shortest way to travel between two distant points is by great circle navigation, as known by ocean navigators for some time. This route shows as curved on any map except for one using a gnomonic projection. Radio waves also follow a great circle, so navies have produced maps using gnomonic projection for use in radio direction finding to locate enemy warships.
Since the 1500s, many people have sailed or flown completely around Earth in all directions, and none have discovered an edge or impenetrable barrier. (See Arctic exploration and History of Antarctica.)
Some flat Earth conjectures that propose that Earth is a north-pole-centered disk conceive of Antarctica as an impenetrable ice wall that encircles the planet and hides any edges. This disk model explains east-west circumnavigation as simply moving around the disk in a circle. (East-west paths form a circle in both disk and spherical geometry.) It is possible in this model to traverse the North Pole, but it would not be possible to perform a circumnavigation that includes the South Pole (which it posits does not exist).
The Arctic Circle is roughly 16,000 km (9,900 mi) long, as is the Antarctic Circle. A "true circumnavigation" of Earth is defined, in order to account for the shape of Earth, to be about 2.5 times as long, including a crossing of the equator, at about 40,000 km (25,000 mi). On the flat Earth model, the ratios would require the Antarctic Circle to be 2.5 times the length of the circumnavigation, or 2.5 × 2.5 = 6.25 times the length of the Arctic Circle.
Explorers, government researchers, commercial pilots, and tourists have been to Antarctica and found that it is not a large ring that encircles the entirety of Earth, but actually a roughly disk-shaped continent smaller than South America but larger than Australia, with an interior that can in fact be traversed in order to take a shorter path from, for example, the tip of South America to Australia than would be possible on a disk.
The first land crossing of the entirety of Antarctica was the Commonwealth Trans-Antarctic Expedition in 1955–1958, and many exploratory airplanes have since passed over the continent in various directions.
== Grid distortion on a spherical surface ==
A meridian of longitude is a line where local solar noon occurs at the same time each day. These lines define "north" and "south". These are perpendicular to lines of latitude that define "east" and "west", where the Sun is at the same angle at local noon on the same day. If the Sun were travelling from east to west over a flat Earth, meridian lines would always be the same distance apart – they would form a square grid when combined with lines of latitude. In reality, meridian lines get farther apart as one travels toward the equator, which is only possible on a round Earth. In places where land is plotted on a grid system, this causes discontinuities in the grid. For example, in areas of the Midwestern United States that use the Public Land Survey System, the northernmost and westernmost sections of a survey township deviate from what would otherwise be an exact square mile. The resulting discontinuities are sometimes reflected directly in local roads, which have kinks where the grid cannot follow completely straight lines. This distortion also affects how aerial photographs taken over large areas can be stitched together.
The Mercator projection has examples of size distortions.
=== Spherical versus flat triangles ===

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Because Earth is spherical, long-distance travel sometimes requires heading in different directions than one would head on a flat Earth. An example would be an airplane travelling 10,000 kilometres (6,200 mi) in a straight line, taking a 90-degree right turn, travelling another 10,000 kilometres (6,200 mi), taking another 90-degree right turn, and travelling 10,000 kilometres (6,200 mi) a third time. On a flat Earth, the aircraft would have travelled along three sides of a square, and arrive at a spot about 10,000 kilometres (6,200 mi) from where it started. But because Earth is spherical, in reality it will have travelled along three sides of a triangle, and arrive back very close to its starting point. If the starting point is the North Pole, it would have travelled due south from the North Pole to the equator, then west for a quarter of the way around Earth, and then due north back to the North Pole.
In spherical geometry, the sum of angles inside a triangle is greater than 180° (in this example 270°, having arrived back at the north pole a 90° angle to the departure path) unlike on a flat surface, where it is always exactly 180°.
== Weather systems ==
Low-pressure weather systems with inward winds (such as a hurricane) spin counterclockwise north of the equator, but clockwise south of the equator. This is due to the Coriolis force, and requires that (assuming they are attached to each other and rotating in the same direction) the north and southern halves of Earth are angled in opposite directions (as in, the north is facing toward Polaris and the south is facing away from it).
== Gravity ==
The laws of gravity, chemistry, and physics that explain the formation and rounding of Earth are well-tested through experiment, and applied successfully to many engineering tasks.
From these laws, the amount of mass Earth contains is known, as is the fact that a non-spherical planet the size of Earth would not be able to support itself against its own gravity. A disk the size of Earth, for example, would likely crack, heat up, liquefy, and re-form into a roughly spherical shape. On a disk strong enough to maintain its shape, gravity would not pull downward with respect to the surface, but would pull toward the center of the disk, contrary to what is observed on level terrain (and which would cause major problems with oceans flowing toward the center of the disk).
Ignoring the other concerns, some flat Earth theorists explain the observed surface "gravity" by proposing that the flat Earth is constantly accelerating upwards. Such a conjecture would also leave open for explanation the tides seen in Earth's oceans, which are conventionally explained by the gravity exerted by the Sun and Moon. The Earth would also quickly approach light-speed in this scenario because the pull of gravity would increase by -9.8m/s, each second (as the formula for gravitational acceleration is measured in m/s2).
== Modern technology ==
Observations of Foucault pendulums, popular in science museums around the world, demonstrate both that the world is spherical and that it rotates (not that the stars are rotating around it).
The mathematics of navigation using Global Positioning System (GPS) satellites assumes that they are moving in known orbits around an approximately spherical surface. The accuracy of GPS navigation in determining latitude and longitude and the way these numbers map onto locations on the ground show that these assumptions are correct. The same is true for the operational GLONASS system run by Russia, the in-development European Galileo, the Chinese BeiDou, and the Indian Regional Navigation Satellite System.
Satellites, including communications satellites used for television, telephone, and Internet connections, would not stay in orbit unless the modern theory of gravitation were correct. The details of which satellites are visible from which places on the ground at which times prove an approximately spherical shape of Earth.
Radio transmitters are mounted on tall towers because they generally rely on line-of-sight propagation. The distance to the horizon is further at higher altitude, so mounting them higher significantly increases the area they can serve. Some signals can be transmitted at much longer distances, but only if they are at frequencies where they can use groundwave propagation, tropospheric propagation, tropospheric scatter, or ionospheric propagation to reflect or refract signals around the curve of Earth.
Equatorial mounts allow astronomers to point telescopes at the same celestial object for longer times while compensating for Earth's rotation in an easy way. The axis of an equatorial mount is parallel to Earth's surface when observing stars at Earth's equator – but perpendicular to it when observing from one of Earth's poles. Equatorial mounts were specifically developed for a spherical and rotating Earth. If Earth were flat, an equatorial mount would not make sense.
Footage of shadows from live webcams can be combined with location and orientation data to locate the sun.
=== Building engineering ===
The design of some large structures needs to take the shape of Earth into account. For example, the towers of the Humber Bridge, although both vertical with respect to gravity, are 36 mm (1.4 inches) farther apart at the top than the bottom due to Earth's curvature.

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=== Aircraft and spacecraft ===
People in high-flying aircraft or skydiving from high-altitude balloons can plainly see the curvature of Earth. Low-flying planes and commercial airliners do not necessarily fly high enough to make this obvious, especially when passenger windows narrow the field of view or clouds or terrain reduce the effective height from the visible surface. Trying to measure the curvature of the horizon by taking a picture is complicated by the fact that both windows and camera lenses can produce distorted images depending on the angle used. An extreme version of this effect can be seen in the fisheye lens. Scientific measurements would require a carefully calibrated lens.
Photos of the ground taken from airplanes over a large enough area also do not fit seamlessly together on a flat surface, but do fit on a roughly spherical surface. Aerial photographs of large areas must be corrected to account for curvature.
Many pictures have been taken of the entirety of Earth by satellites launched by a variety of governments and private organizations. From high orbits, where half the planet can be seen at once, it is plainly spherical. The only way to piece together all the pictures taken of the ground from lower orbits so that all the surface features line up seamlessly and without distortion is to put them on an approximately spherical surface.
Astronauts in low Earth orbit can personally see the curvature of the planet, and travel all the way around several times a day. The astronauts who travelled to the Moon have seen the entire Moon-facing half at once, and can watch the sphere rotate once a day (approximately; the Moon is also moving with respect to Earth).
When the supersonic aircraft Concorde took off not long after sunset from London and flew westward to New York, it outran the Sun's apparent motion westward – and therefore passengers aboard observed the Sun rising in the west as they travelled. After landing in New York, passengers watched a second sunset in the west.
Because the speed of the Sun's shadow is slower in polar regions (due to the steeper angle), even a subsonic aircraft can overtake the sunset when flying at high latitudes. One photographer used a roughly circular route around the North Pole to take pictures of 24 sunsets in the same 24-hour period, pausing westward progress in each time zone to let the shadow of the Sun catch up. The surface of Earth rotates at 180.17 miles per hour (289.96 km/h) at 80° north or south, and 1,040.4 miles per hour (1,674.4 km/h) at the equator.
=== Ring-laser gyroscope ===
In the documentary Behind the Curve, Bob Knodel uses a ring-laser gyroscope to attempt to prove that the earth does not rotate. The results instead showed a 15 degree per hour drift, due to the earth's rotation.
== See also ==
Earth ellipsoid
Geodesy
Spherical Earth
Timeline of Earth estimates
== References ==
== External links ==

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Evidence for a proposition is what supports the proposition. It is usually understood as an indication that the proposition is true. The exact definition and role of evidence vary across different fields.
In epistemology, evidence is what justifies beliefs or what makes it rational to hold a certain doxastic attitude. For example, a perceptual experience of a tree may serve as evidence to justify the belief that there is a tree. In this role, evidence is usually understood as a private mental state. In phenomenology, evidence is limited to intuitive knowledge, often associated with the controversial assumption that it provides indubitable access to truth.
In science, scientific evidence is information gained through the scientific method that confirms or disconfirms scientific hypotheses, acting as a neutral arbiter between competing theories. Measurements of Mercury's "anomalous" orbit, for example, are seen as evidence that confirms Einstein's theory of general relativity. The problems of underdetermination and theory-ladenness are two obstacles that threaten to undermine the role of scientific evidence. Philosophers of science tend to understand evidence not as mental states but as verifiable information, observable physical objects or events, secured by following the scientific method.
In law, evidence is information to establish or refute claims relevant to a case, such as testimony, documentary evidence, and physical evidence.
The relation between evidence and a supported statement can vary in strength, ranging from weak correlation to indisputable proof. Theories of the evidential relation examine the nature of this connection. Probabilistic approaches hold that something counts as evidence if it increases the probability of the supported statement. According to hypothetico-deductivism, evidence consists in observational consequences of a hypothesis. The positive-instance approach states that an observation sentence is evidence for a universal statement if the sentence describes a positive instance of this statement.
== Philosophy of evidence ==
=== Characteristics ===
Understood in its broadest sense, evidence for a proposition is what supports this proposition. Traditionally, the term is sometimes understood in a narrower sense: as the intuitive knowledge of facts that are considered indubitable. In this sense, only the singular form is used. This meaning is found especially in phenomenology, in which evidence is elevated to one of the basic principles of philosophy, giving philosophy the ultimate justifications that are supposed to turn it into a rigorous science. In a more modern usage, the plural form is also used. In academic discourse, evidence plays a central role in epistemology and in the philosophy of science. Reference to evidence is made in many different fields, like in science, in the legal system, in history, in journalism and in everyday discourse. A variety of different attempts have been made to conceptualize the nature of evidence. These attempts often proceed by starting with intuitions from one field or in relation to one theoretical role played by evidence and go on to generalize these intuitions, leading to a universal definition of evidence.
One important intuition is that evidence is what justifies beliefs. This line of thought is usually followed in epistemology and tends to explain evidence in terms of private mental states, for example, as experiences, other beliefs or knowledge. This is closely related to the idea that how rational someone is, is determined by how they respond to evidence. Another intuition, which is more dominant in the philosophy of science, focuses on evidence as that which confirms scientific hypotheses and arbitrates between competing theories. On this view, it is essential that evidence is public so that different scientists can share the same evidence. This leaves publicly observable phenomena like physical objects and events as the best candidates for evidence, unlike private mental states. One problem with these approaches is that the resulting definitions of evidence, both within a field and between fields, vary a lot and are incompatible with each other. For example, it is not clear what a bloody knife and a perceptual experience have in common when both are treated as evidence in different disciplines. This suggests that there is no unitary concept corresponding to the different theoretical roles ascribed to evidence, i.e. that we do not always mean the same thing when we talk of evidence.
On the other hand, Aristotle, phenomenologists, and numerous scholars accept that there could be several degrees of evidence. For instance, while the outcome of a complex equation may become more or less evident to a mathematician after hours of deduction, yet with little doubts about it, a simpler formula would appear more evident to them.
Riofrio has detected some characteristics that are present in evident arguments and proofs. The more they are evident, the more these characteristics will be present. There are six intrinsic characteristics of evidence:
The truth lies in what is evident, while falsehood or irrationality, although it may appear evident at times, lacks true evidence.
What is evident aligns coherently with other truths acquired through knowledge. Any insurmountable incoherence would indicate the presence of error or falsehood.
Evident truths are based on necessary reasoning.
The simplest truths are the most evident. They are self-explanatory and do not require argumentation to be understood by the intellect. However, for those lacking education, certain complex truths require rational discourse to become evident.
Evident truths do not need justification; they are indubitable. They are intuitively grasped by the intellect, without the need for further discourse, arguments, or proof.
Evident truths are clear, translucent, and filled with light.
In addition, four subjective or external characteristics can be detected over those things that are more or less evident:

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The evident instills certainty and grants the knower a subjective sense of security, as they believe to have aligned with the truth
Initially, evident truths are perceived as natural and effortless, as Aristotle highlighted. They are innately present within the intellect, fostering a peaceful and harmonious understanding.
Consequently, evident truths appear to be widely shared, strongly connected to common sense, which comprises generally accepted beliefs.
Evident truths are fertile ground: they provide a solid foundation for other branches of scientific knowledge to flourish.
These ten characteristics of what is evident allowed Riofrio to formulate a test of evidence to detect the level of certainty or evidence that one argument or proof could have.
=== Evidential relation ===
Philosophers in the 20th century started to investigate the "evidential relation", the relation between evidence and the proposition supported by it. The issue of the nature of the evidential relation concerns the question of what this relation has to be like in order for one thing to justify a belief or to confirm a hypothesis. Important theories in this field include the probabilistic approach, hypothetico-deductivism and the positive-instance approach.
Probabilistic approaches, also referred to as Bayesian confirmation theory, explain the evidential relation in terms of probabilities. They hold that all that is necessary is that the existence of the evidence increases the likelihood that the hypothesis is true. This can be expressed mathematically as
P
(
H
∣
E
)
>
P
(
H
)
{\displaystyle P(H\mid E)>P(H)}
. In words: a piece of evidence (E) confirms a hypothesis (H) if the conditional probability of this hypothesis relative to the evidence is higher than the unconditional probability of the hypothesis by itself. Smoke (E), for example, is evidence that there is a fire (H), because the two usually occur together, which is why the likelihood of fire given that there is smoke is higher than the likelihood of fire by itself. On this view, evidence is akin to an indicator or a symptom of the truth of the hypothesis. Against this approach, it has been argued that it is too liberal because it allows accidental generalizations as evidence. Finding a nickel in one's pocket, for example, raises the probability of the hypothesis that "All the coins in my pockets are nickels". But, according to Alvin Goldman, it should not be considered evidence for this hypothesis since there is no lawful connection between this one nickel and the other coins in the pocket.
Hypothetico-deductivism is a non-probabilistic approach that characterizes the evidential relations in terms of deductive consequences of the hypothesis. According to this view, "evidence for a hypothesis is a true observational consequence of that hypothesis". One problem with the characterization so far is that hypotheses usually contain relatively little information and therefore have few if any deductive observational consequences. So the hypothesis by itself that there is a fire does not entail that smoke is observed. Instead, various auxiliary assumptions have to be included about the location of the smoke, the fire, the observer, the lighting conditions, the laws of chemistry, etc. In this way, the evidential relation becomes a three-place relation between evidence, hypothesis and auxiliary assumptions. This means that whether a thing is evidence for a hypothesis depends on the auxiliary assumptions one holds. This approach fits well with various scientific practices. For example, it is often the case that experimental scientists try to find evidence that would confirm or disconfirm a proposed theory. The hypothetico-deductive approach can be used to predict what should be observed in an experiment if the theory was true. It thereby explains the evidential relation between the experiment and the theory. One problem with this approach is that it cannot distinguish between relevant and certain irrelevant cases. So if smoke is evidence for the hypothesis "there is fire", then it is also evidence for conjunctions including this hypothesis, for example, "there is fire and Socrates was wise", despite the fact that Socrates's wisdom is irrelevant here.
According to the positive-instance approach, an observation sentence is evidence for a universal hypothesis if the sentence describes a positive instance of this hypothesis. For example, the observation that "this swan is white" is an instance of the universal hypothesis that "all swans are white". This approach can be given a precise formulation in first-order logic: a proposition is evidence for a hypothesis if it entails the "development of the hypothesis". Intuitively, the development of the hypothesis is what the hypothesis states if it was restricted to only the individuals mentioned in the evidence. In the case above, we have the hypothesis "
∀
x
(
s
w
a
n
(
x
)
→
w
h
i
t
e
(
x
)
)
{\displaystyle \forall x(swan(x)\rightarrow white(x))}
" (all swans are white) which, when restricted to the domain "{a}", containing only the one individual mentioned in the evidence, entails the evidence, i.e. "
s
w
a
n
(
a
)
∧
w
h
i
t
e
(
a
)
{\displaystyle swan(a)\land white(a)}
" (this swan is white). One important shortcoming of this approach is that it requires that the hypothesis and the evidence are formulated in the same vocabulary, i.e. use the same predicates, like "
s
w
a
n
{\displaystyle swan}
" or "
w
h
i
t
e
{\displaystyle white}
" above. But many scientific theories posit theoretical objects, like electrons or strings in physics, that are not directly observable and therefore cannot show up in the evidence as conceived here.
=== In specific fields ===
Important theorists of evidence include Bertrand Russell, Willard Van Orman Quine, the logical positivists, Timothy Williamson, Earl Conee and Richard Feldman. Russell, Quine and the logical positivists belong to the empiricist tradition and hold that evidence consists in sense data, stimulation of one's sensory receptors and observation statements, respectively. According to Williamson, all and only knowledge constitute evidence. Conee and Feldman hold that only one's current mental states should be considered evidence.

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==== In epistemology ====
The guiding intuition within epistemology concerning the role of evidence is that it is what justifies beliefs. For example, Phoebe's auditory experience of the music justifies her belief that the speakers are on. Evidence has to be possessed by the believer in order to play this role. So Phoebe's own experiences can justify her own beliefs but not someone else's beliefs. Some philosophers hold that evidence possession is restricted to conscious mental states, for example, to sense data. This view has the implausible consequence that many of simple everyday-beliefs would be unjustified. The more common view is that all kinds of mental states, including stored beliefs that are currently unconscious, can act as evidence. It is sometimes argued that the possession of a mental state capable of justifying another is not sufficient for the justification to happen. The idea behind this line of thought is that justified belief has to be connected to or grounded in the mental state acting as its evidence. So Phoebe's belief that the speakers are on is not justified by her auditory experience if the belief is not based in this experience. This would be the case, for example, if Phoebe has both the experience and the belief but is unaware of the fact that the music is produced by the speakers.
It is sometimes held that only propositional mental states can play this role, a position known as "propositionalism". A mental state is propositional if it is an attitude directed at a propositional content. Such attitudes are usually expressed by verbs like "believe" together with a that-clause, as in "Robert believes that the corner shop sells milk". Such a view denies that sensory impressions can act as evidence. This is often held as an argument against this view since sensory impressions are commonly treated as evidence. Propositionalism is sometimes combined with the view that only attitudes to true propositions can count as evidence. On this view, the belief that the corner shop sells milk only constitutes evidence for the belief that the corner shop sells dairy products if the corner shop actually sells milk. Against this position, it has been argued that evidence can be misleading but still count as evidence.
This line of thought is often combined with the idea that evidence, propositional or otherwise, determines what it is rational for us to believe. But it can be rational to have a false belief. This is the case when we possess misleading evidence. For example, it was rational for Neo in the Matrix movie to believe that he was living in the 20th century because of all the evidence supporting his belief despite the fact that this evidence was misleading since it was part of a simulated reality. This account of evidence and rationality can also be extended to other doxastic attitudes, like disbelief and suspension of belief. So rationality does not just demand that we believe something if we have decisive evidence for it, it also demands that we disbelieve something if we have decisive evidence against it and that we suspend belief if we lack decisive evidence either way.
==== In phenomenology ====
The meaning of the term "evidence" in phenomenology shows many parallels to its epistemological usage, but it is understood in a narrower sense. Thus, evidence here specifically refers to intuitive knowledge, which is described as "self-given" (selbst-gegeben). This contrasts with empty intentions, in which one refers to states of affairs through a certain opinion, but without an intuitive presentation. This is why evidence is often associated with the controversial thesis that it constitutes an immediate access to truth. In this sense, the evidently given phenomenon guarantees its own truth and is therefore considered indubitable. Due to this special epistemological status of evidence, it is regarded in phenomenology as the basic principle of all philosophy. In this form, it represents the lowest foundation of knowledge, which consists of indubitable insights upon which all subsequent knowledge is built. This evidence-based method is meant to make it possible for philosophy to overcome many of the traditionally unresolved disagreements and thus become a rigorous science. This far-reaching claim of phenomenology, based on absolute certainty, is one of the focal points of criticism by its opponents. Thus, it has been argued that even knowledge based on self-evident intuition is fallible. This can be seen, for example, in the fact that even among phenomenologists, there is much disagreement about the basic structures of experience.

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==== In philosophy of science ====
In the sciences, evidence is understood as what confirms or disconfirms scientific hypotheses. The term "confirmation" is sometimes used synonymously with that of "evidential support". Measurements of Mercury's "anomalous" orbit, for example, are seen as evidence that confirms Einstein's theory of general relativity. This is especially relevant for choosing between competing theories. So in the case above, evidence plays the role of neutral arbiter between Newton's and Einstein's theory of gravitation. This is only possible if scientific evidence is public and uncontroversial so that proponents of competing scientific theories agree on what evidence is available. These requirements suggest scientific evidence consists not of private mental states but of public physical objects or events.
It is often held that evidence is in some sense prior to the hypotheses it confirms. This was sometimes understood as temporal priority, i.e. that we come first to possess the evidence and later form the hypothesis through induction. But this temporal order is not always reflected in scientific practice, where experimental researchers may look for a specific piece of evidence in order to confirm or disconfirm a pre-existing hypothesis. Logical positivists, on the other hand, held that this priority is semantic in nature, i.e. that the meanings of the theoretical terms used in the hypothesis are determined by what would count as evidence for them. Counterexamples for this view come from the fact that our idea of what counts as evidence may change while the meanings of the corresponding theoretical terms remain constant. The most plausible view is that this priority is epistemic in nature, i.e. that our belief in a hypothesis is justified based on the evidence while the justification for the belief in the evidence does not depend on the hypothesis.
A central issue for the scientific conception of evidence is the problem of underdetermination, i.e. that the evidence available supports competing theories equally well. So, for example, evidence from our everyday life about how gravity works confirms Newton's and Einstein's theory of gravitation equally well and is therefore unable to establish consensus among scientists. But in such cases, it is often the gradual accumulation of evidence that eventually leads to an emerging consensus. This evidence-driven process towards consensus seems to be one hallmark of the sciences not shared by other fields.
Another problem for the conception of evidence in terms of confirmation of hypotheses is that what some scientists consider the evidence to be may already involve various theoretical assumptions not shared by other scientists. This phenomenon is known as theory-ladenness. Some cases of theory-ladenness are relatively uncontroversial, for example, that the numbers output by a measurement device need additional assumptions about how this device works and what was measured in order to count as meaningful evidence. Other putative cases are more controversial, for example, the idea that different people or cultures perceive the world through different, incommensurable conceptual schemes, leading them to very different impressions about what is the case and what evidence is available. Theory-ladenness threatens to impede the role of evidence as neutral arbiter since these additional assumptions may favor some theories over others. It could thereby also undermine a consensus to emerge since the different parties may be unable to agree even on what the evidence is. When understood in the widest sense, it is not controversial that some form of theory-ladenness exists. But it is questionable whether it constitutes a serious threat to scientific evidence when understood in this sense.
== Different types of evidence ==
=== In science (empirical evidence) ===
In scientific research evidence is accumulated through observations of phenomena that occur in the natural world, or which are created as experiments in a laboratory or other controlled conditions. Scientists tend to focus on how the data used during statistical inference are generated. Scientific evidence usually goes towards supporting or rejecting a hypothesis.
The burden of proof is on the person making a contentious claim. Within science, this translates to the burden resting on presenters of a paper, in which the presenters argue for their specific findings. This paper is placed before a panel of judges where the presenter must defend the thesis against all challenges.
When evidence is contradictory to predicted expectations, the evidence and the ways of making it are often closely scrutinized (see experimenter's regress) and only at the end of this process is the hypothesis rejected: this can be referred to as 'refutation of the hypothesis'. The rules for evidence used by science are collected systematically in an attempt to avoid the bias inherent to anecdotal evidence.
=== In law ===

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In law, the production and presentation of evidence depend first on establishing on whom the burden of proof lies. Admissible evidence is that which a court receives and considers for the purposes of deciding a particular case. Two primary burden-of-proof considerations exist in law. The first is on whom the burden rests. In many, especially Western, courts, the burden of proof is placed on the prosecution in criminal cases and the plaintiff in civil cases. The second consideration is the degree of certitude proof must reach, depending on both the quantity and quality of evidence. These degrees are different for criminal and civil cases, the former requiring evidence beyond a reasonable doubt, the latter considering only which side has the preponderance of evidence, or whether the proposition is more likely true or false.
The parts of a legal case that are not in controversy are known, in general, as the "facts of the case." Beyond any facts that are undisputed, a judge or jury is usually tasked with being a trier of fact for the other issues of a case. Evidence and rules are used to decide questions of fact that are disputed, some of which may be determined by the legal burden of proof relevant to the case. Evidence in certain cases (e.g. capital crimes) must be more compelling than in other situations (e.g. minor civil disputes), which drastically affects the quality and quantity of evidence necessary to decide a case. The decision-maker, often a jury, but sometimes a judge decides whether the burden of proof has been fulfilled. After deciding who will carry the burden of proof, the evidence is first gathered and then presented before the court:
==== Collection ====
In a criminal investigation, rather than attempting to prove an abstract or hypothetical point, the evidence gatherers attempt to determine who is responsible for a criminal act. The focus of criminal evidence is to connect physical evidence and reports of witnesses to a specific person.
==== Presentation ====
The path that physical evidence takes from the scene of a crime or the arrest of a suspect to the courtroom is called the chain of custody. In a criminal case, this path must be clearly documented or attested to by those who handled the evidence. If the chain of evidence is broken, a defendant may be able to persuade the judge to declare the evidence inadmissible.
Presenting evidence before the court differs from the gathering of evidence in important ways. Gathering evidence may take many forms; presenting evidence that tends to prove or disprove the point at issue is strictly governed by rules. Failure to follow these rules leads to any number of consequences. In law, certain policies allow (or require) evidence to be excluded from consideration based either on indicia relating to reliability, or broader social concerns. Testimony (which tells) and exhibits (which show) are the two main categories of evidence presented at a trial or hearing. In the United States, evidence in federal court is admitted or excluded under the Federal Rules of Evidence.
==== Burden of proof ====
The burden of proof is the obligation of a party in an argument or dispute to provide sufficient evidence to shift the other party's or a third party's belief from their initial position. The burden of proof must be fulfilled by both establishing confirming evidence and negating oppositional evidence. Conclusions drawn from evidence may be subject to criticism based on a perceived failure to fulfill the burden of proof.
Two principal considerations are:
On whom does the burden of proof rest?
To what degree of certitude must the assertion be supported?
The latter question depends on the nature of the point under contention and determines the quantity and quality of evidence required to meet the burden of proof.
In a criminal trial in the United States, for example, the prosecution carries the burden of proof since the defendant is presumed innocent until proven guilty beyond a reasonable doubt. Similarly, in most civil procedures, the plaintiff carries the burden of proof and must convince a judge or jury that the preponderance of the evidence is on their side. Other legal standards of proof include "reasonable suspicion", "probable cause" (as for arrest), "prima facie evidence", "credible evidence", "substantial evidence", and "clear and convincing evidence".
In a philosophical debate, there is an implicit burden of proof on the party asserting a claim, since the default position is generally one of neutrality or unbelief. Each party in a debate will therefore carry the burden of proof for any assertion they make in the argument, although some assertions may be granted by the other party without further evidence. If the debate is set up as a resolution to be supported by one side and refuted by another, the overall burden of proof is on the side supporting the resolution.
==== Specific types ====
Digital evidence
Physical evidence
Relationship evidence
Testimonial evidence
Trace evidence
== See also ==
== References ==
== External links ==
Evidence at PhilPapers
Zalta, Edward N. (ed.). "Evidence". Stanford Encyclopedia of Philosophy. ISSN 1095-5054. OCLC 429049174.
Fieser, James; Dowden, Bradley (eds.). "Evidence". Internet Encyclopedia of Philosophy. ISSN 2161-0002. OCLC 37741658.
Evidence at the Indiana Philosophy Ontology Project
ASTM E141 Standard Practice for Acceptance of Evidence Based on the Results of Probability Sampling
"Evidence" . Encyclopædia Britannica (11th ed.). 1911.

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Evidence-based medicine (EBM), sometimes known within healthcare as evidence-based practice (EBP), is "the conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients. It means integrating individual clinical expertise with the best available external clinical evidence from systematic research." The aim of EBM is to integrate the experience of the clinician, the values of the patient, and the best available scientific information to guide decision-making about clinical management. The term was originally used to describe an approach to teaching the practice of medicine and improving decisions by individual physicians about individual patients.
The EBM Pyramid is a tool that helps in visualizing the hierarchy of evidence in medicine, from least authoritative, like expert opinions, to most authoritative, like systematic reviews.
Adoption of evidence-based medicine is necessary in a human rights-based approach to public health and a precondition for accessing the right to health.
== Background, history, and definition ==
Medicine has a long history of scientific inquiry into the prevention, diagnosis, and treatment of human disease. In the 11th century AD, Avicenna, a Persian physician and philosopher, developed an approach to EBM that was mostly similar to current ideas and practises.
The concept of a controlled clinical trial was first described in 1662 by Jan Baptist van Helmont in reference to the practice of bloodletting. Wrote Van Helmont:
Let us take out of the Hospitals, out of the Camps, or from elsewhere, 200, or 500 poor People, that have fevers or Pleuritis. Let us divide them in Halfes, let us cast lots, that one halfe of them may fall to my share, and the others to yours; I will cure them without blood-letting and sensible evacuation; but you do, as ye know ... we shall see how many Funerals both of us shall have...
The first published report describing the conduct and results of a controlled clinical trial was by James Lind, a Scottish naval surgeon who conducted research on scurvy during his time aboard HMS Salisbury in the Channel Fleet, while patrolling the Bay of Biscay. Lind divided the sailors participating in his experiment into six groups, so that the effects of various treatments could be fairly compared. Lind found improvement in symptoms and signs of scurvy among the group of men treated with lemons or oranges. He published a treatise describing the results of this experiment in 1753.
An early critique of statistical methods in medicine was published in 1835, in Comtes Rendus de l'Académie des Sciences, Paris, by a man referred to as "Mr Civiale".
In 1990, Gordon Guyatt, then a young internal medicine residency coordinator at McMaster University, introduced a teaching method he initially termed "Scientific Medicine." This approach emphasized applying critical appraisal techniques directly to bedside clinical decision-making, building on the work of his mentor, David Sackett. However, the concept met resistance from colleagues, as it implied that existing clinical practices lacked scientific rigor, even though this was likely true. To address this, Guyatt rebranded the approach as "Evidence-Based Medicine", a term first formally introduced in a 1991 editorial in the ACP Journal Club. Although the name was coined in 1991, it took several years after and a concerted efforts of many other teams to define the foundations of this method.
Although more popular in medicine, the concept of "evidence-based" is spreading to other disciplines, such as the humanities, and to languages other than English, albeit at a slower pace.
=== Clinical decision-making ===
Alvan Feinstein's publication of Clinical Judgment in 1967 focused attention on the role of clinical reasoning and identified biases that can affect it. In 1972, Archie Cochrane published Effectiveness and Efficiency, which described the lack of controlled trials supporting many practices that had previously been assumed to be effective. In 1973, John Wennberg began to document wide variations in how physicians practiced. Through the 1980s, David M. Eddy described errors in clinical reasoning and gaps in evidence. In the mid-1980s, Alvin Feinstein, David Sackett and others published textbooks on clinical epidemiology, which translated epidemiological methods to physician decision-making. Toward the end of the 1980s, a group at RAND showed that large proportions of procedures performed by physicians were considered inappropriate even by the standards of their own experts.
=== Evidence-based guidelines and policies ===
David M. Eddy first began to use the term 'evidence-based' in 1987 in workshops and a manual commissioned by the Council of Medical Specialty Societies to teach formal methods for designing clinical practice guidelines. The manual was eventually published by the American College of Physicians. Eddy first published the term 'evidence-based' in March 1990, in an article in the Journal of the American Medical Association (JAMA) that laid out the principles of evidence-based guidelines and population-level policies, which Eddy described as "explicitly describing the available evidence that pertains to a policy and tying the policy to evidence instead of standard-of-care practices or the beliefs of experts. The pertinent evidence must be identified, described, and analyzed. The policymakers must determine whether the policy is justified by the evidence. A rationale must be written." He discussed evidence-based policies in several other papers published in JAMA in the spring of 1990. Those papers were part of a series of 28 published in JAMA between 1990 and 1997 on formal methods for designing population-level guidelines and policies.

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=== Medical education ===
The term 'evidence-based medicine' was introduced slightly later, in the context of medical education. In the autumn of 1990, Gordon Guyatt used it in an unpublished description of a program at McMaster University for prospective or new medical students. Guyatt and others first published the term two years later (1992) to describe a new approach to teaching the practice of medicine.
In 1996, David Sackett and colleagues clarified the definition of this tributary of evidence-based medicine as "the conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients. ... [It] means integrating individual clinical expertise with the best available external clinical evidence from systematic research." This branch of evidence-based medicine aims to make individual decision making more structured and objective by better reflecting the evidence from research. Population-based data are applied to the care of an individual patient, while respecting the fact that practitioners have clinical expertise reflected in effective and efficient diagnosis and thoughtful identification and compassionate use of individual patients' predicaments, rights, and preferences.
Between 1993 and 2000, the Evidence-Based Medicine Working Group at McMaster University published the methods to a broad physician audience in a series of 25 "Users' Guides to the Medical Literature" in JAMA. In 1995 Rosenberg and Donald defined individual-level, evidence-based medicine as "the process of finding, appraising, and using contemporaneous research findings as the basis for medical decisions." In 2010, Greenhalgh used a definition that emphasized quantitative methods: "the use of mathematical estimates of the risk of benefit and harm, derived from high-quality research on population samples, to inform clinical decision-making in the diagnosis, investigation or management of individual patients."
The two original definitions highlight important differences in how evidence-based medicine is applied to populations versus individuals. When designing guidelines applied to large groups of people in settings with relatively little opportunity for modification by individual physicians, evidence-based policymaking emphasizes that good evidence should exist to document a test's or treatment's effectiveness. In the setting of individual decision-making, practitioners can be given greater latitude in how they interpret research and combine it with their clinical judgment. In 2005, Eddy offered an umbrella definition for the two branches of EBM: "Evidence-based medicine is a set of principles and methods intended to ensure that to the greatest extent possible, medical decisions, guidelines, and other types of policies are based on and consistent with good evidence of effectiveness and benefit."
=== Progress ===
In the area of evidence-based guidelines and policies, the explicit insistence on evidence of effectiveness was introduced by the American Cancer Society in 1980. The U.S. Preventive Services Task Force (USPSTF) began issuing guidelines for preventive interventions based on evidence-based principles in 1984. In 1985, the Blue Cross Blue Shield Association applied strict evidence-based criteria for covering new technologies. Beginning in 1987, specialty societies such as the American College of Physicians, and voluntary health organizations such as the American Heart Association, wrote many evidence-based guidelines. In 1991, Kaiser Permanente, a managed care organization in the US, began an evidence-based guidelines program. In 1991, Richard Smith wrote an editorial in the British Medical Journal and introduced the ideas of evidence-based policies in the UK. In 1993, the Cochrane Collaboration created a network of 13 countries to produce systematic reviews and guidelines. In 1997, the US Agency for Healthcare Research and Quality (AHRQ, then known as the Agency for Health Care Policy and Research, or AHCPR) established Evidence-based Practice Centers (EPCs) to produce evidence reports and technology assessments to support the development of guidelines. In the same year, a National Guideline Clearinghouse that followed the principles of evidence-based policies was created by AHRQ, the AMA, and the American Association of Health Plans (now America's Health Insurance Plans). In 1999, the National Institute for Clinical Excellence (NICE) was created in the UK to circulate evidence and guidance on treatments within the NHS.
In the area of medical education, medical schools in Canada, the US, the UK, Australia, and other countries now offer programs that teach evidence-based medicine. A 2009 study of UK programs found that more than half of UK medical schools offered some training in evidence-based medicine, although the methods and content varied considerably, and EBM teaching was restricted by lack of curriculum time, trained tutors and teaching materials. Many programs have been developed to help individual physicians gain better access to evidence. For example, UpToDate was created in the early 1990s. The Cochrane Collaboration began publishing evidence reviews in 1993. In 1995, BMJ Publishing Group launched Clinical Evidence, a 6-monthly periodical that provided brief summaries of the current state of evidence about important clinical questions for clinicians.
=== Current practice ===
By 2000, use of the term evidence-based had extended to other levels of the health care system. An example is evidence-based health services, which seek to increase the competence of health service decision makers and the practice of evidence-based medicine at the organizational or institutional level.
The multiple tributaries of evidence-based medicine share an emphasis on the importance of incorporating evidence from formal research in medical policies and decisions. However, because they differ on the extent to which they require good evidence of effectiveness before promoting a guideline or payment policy, a distinction is sometimes made between evidence-based medicine and science-based medicine, which also takes into account factors such as prior plausibility and compatibility with established science (as when medical organizations promote controversial treatments such as acupuncture). Differences also exist regarding the extent to which it is feasible to incorporate individual-level information in decisions. Thus, evidence-based guidelines and policies may not readily "hybridise" with experience-based practices orientated towards ethical clinical judgement, and can lead to contradictions, contest, and unintended crises. The most effective "knowledge leaders" (managers and clinical leaders) use a broad range of management knowledge in their decision making, rather than just formal evidence. Evidence-based guidelines may provide the basis for governmentality in health care, and consequently play a central role in the governance of contemporary health care systems.

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== Methods ==
=== Steps ===
The steps for designing explicit, evidence-based guidelines were described in the late 1980s: formulate the question (population, intervention, comparison intervention, outcomes, time horizon, setting); search the literature to identify studies that inform the question; interpret each study to determine precisely what it says about the question; if several studies address the question, synthesize their results (meta-analysis); summarize the evidence in evidence tables; compare the benefits, harms and costs in a balance sheet; draw a conclusion about the preferred practice; write the guideline; write the rationale for the guideline; have others review each of the previous steps; implement the guideline.
For the purposes of medical education and individual-level decision making, five steps of EBM in practice were described in 1992 and the experience of delegates attending the 2003 Conference of Evidence-Based Health Care Teachers and Developers was summarized into five steps and published in 2005. This five-step process can broadly be categorized as follows:
Translation of uncertainty to an answerable question; includes critical questioning, study design and levels of evidence
Systematic retrieval of the best evidence available
Critical appraisal of evidence for internal validity that can be broken down into aspects regarding:
Systematic errors as a result of selection bias, information bias and confounding
Quantitative aspects of diagnosis and treatment
The effect size and aspects regarding its precision
Clinical importance of results
External validity or generalizability
Application of results in practice
Evaluation of performance
=== Evidence reviews ===
Systematic reviews of published research studies are a major part of the evaluation of particular treatments. The Cochrane Collaboration is one of the best-known organisations that conducts systematic reviews. Like other producers of systematic reviews, it requires authors to provide a detailed study protocol as well as a reproducible plan of their literature search and evaluations of the evidence. After the best evidence is assessed, treatment is categorized as (1) likely to be beneficial, (2) likely to be harmful, or (3) without evidence to support either benefit or harm.
A 2007 analysis of 1,016 systematic reviews from all 50 Cochrane Collaboration Review Groups found that 44% of the reviews concluded that the intervention was likely to be beneficial, 7% concluded that the intervention was likely to be harmful, and 49% concluded that evidence did not support either benefit or harm. 96% recommended further research. In 2017, a study assessed the role of systematic reviews produced by Cochrane Collaboration to inform US private payers' policymaking; it showed that although the medical policy documents of major US private payers were informed by Cochrane systematic reviews, there was still scope to encourage the further use.
=== Assessing the quality of evidence ===
Evidence-based medicine categorizes different types of clinical evidence and rates or grades them according to the strength of their freedom from the various biases that beset medical research. For example, the strongest evidence for therapeutic interventions is provided by systematic review of randomized, well-blinded, placebo-controlled trials with allocation concealment and complete follow-up involving a homogeneous patient population and medical condition. In contrast, patient testimonials, case reports, and even expert opinion have little value as proof because of the placebo effect, the biases inherent in observation and reporting of cases, and difficulties in ascertaining who is an expert (however, some critics have argued that expert opinion "does not belong in the rankings of the quality of empirical evidence because it does not represent a form of empirical evidence" and continue that "expert opinion would seem to be a separate, complex type of knowledge that would not fit into hierarchies otherwise limited to empirical evidence alone.").
Several organizations have developed grading systems for assessing the quality of evidence. For example, in 1989 the U.S. Preventive Services Task Force (USPSTF) put forth the following system:

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Level I: Evidence obtained from at least one properly designed randomized controlled trial.
Level II-1: Evidence obtained from well-designed controlled trials without randomization.
Level II-2: Evidence obtained from well-designed cohort studies or case-control studies, preferably from more than one center or research group.
Level II-3: Evidence obtained from multiple time series designs with or without the intervention. Dramatic results in uncontrolled trials might also be regarded as this type of evidence.
Level III: Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees.
Another example are the Oxford CEBM Levels of Evidence published by the Centre for Evidence-Based Medicine. First released in September 2000, the Levels of Evidence provide a way to rank evidence for claims about prognosis, diagnosis, treatment benefits, treatment harms, and screening, which most grading schemes do not address. The original CEBM Levels were Evidence-Based On Call to make the process of finding evidence feasible and its results explicit. In 2011, an international team redesigned the Oxford CEBM Levels to make them more understandable and to take into account recent developments in evidence ranking schemes. The Oxford CEBM Levels of Evidence have been used by patients and clinicians, as well as by experts to develop clinical guidelines, such as recommendations for the optimal use of phototherapy and topical therapy in psoriasis and guidelines for the use of the BCLC staging system for diagnosing and monitoring hepatocellular carcinoma in Canada.
In 2000, a system was developed by the Grading of Recommendations Assessment, Development and Evaluation (GRADE) working group. The GRADE system takes into account more dimensions than just the quality of medical research. It requires users who are performing an assessment of the quality of evidence, usually as part of a systematic review, to consider the impact of different factors on their confidence in the results. Authors of GRADE tables assign one of four levels to evaluate the quality of evidence, on the basis of their confidence that the observed effect (a numeric value) is close to the true effect. The confidence value is based on judgments assigned in five different domains in a structured manner. The GRADE working group defines 'quality of evidence' and 'strength of recommendations' based on the quality as two different concepts that are commonly confused with each other.
Systematic reviews may include randomized controlled trials that have low risk of bias, or observational studies that have high risk of bias. In the case of randomized controlled trials, the quality of evidence is high but can be downgraded in five different domains.
Risk of bias: A judgment made on the basis of the chance that bias in included studies has influenced the estimate of effect.
Imprecision: A judgment made on the basis of the chance that the observed estimate of effect could change completely.
Indirectness: A judgment made on the basis of the differences in characteristics of how the study was conducted and how the results are actually going to be applied.
Inconsistency: A judgment made on the basis of the variability of results across the included studies.
Publication bias: A judgment made on the basis of the question whether all the research evidence has been taken to account.
In the case of observational studies per GRADE, the quality of evidence starts off lower and may be upgraded in three domains in addition to being subject to downgrading.
Large effect: Methodologically strong studies show that the observed effect is so large that the probability of it changing completely is less likely.
Plausible confounding would change the effect: Despite the presence of a possible confounding factor that is expected to reduce the observed effect, the effect estimate still shows significant effect.
Dose response gradient: The intervention used becomes more effective with increasing dose. This suggests that a further increase will likely bring about more effect.
Meaning of the levels of quality of evidence as per GRADE:
High Quality Evidence: The authors are very confident that the presented estimate lies very close to the true value. In other words, the probability is very low that further research will completely change the presented conclusions.
Moderate Quality Evidence: The authors are confident that the presented estimate lies close to the true value, but it is also possible that it may be substantially different. In other words, further research may completely change the conclusions.
Low Quality Evidence: The authors are not confident in the effect estimate, and the true value may be substantially different. In other words, further research is likely to change the presented conclusions completely.
Very Low Quality Evidence: The authors do not have any confidence in the estimate and it is likely that the true value is substantially different from it. In other words, new research will probably change the presented conclusions completely.
=== Categories of recommendations ===
In guidelines and other publications, recommendation for a clinical service is classified by the balance of risk versus benefit and the level of evidence on which this information is based. The U.S. Preventive Services Task Force uses the following system:

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Level A: Good scientific evidence suggests that the benefits of the clinical service substantially outweigh the potential risks. Clinicians should discuss the service with eligible patients.
Level B: At least fair scientific evidence suggests that the benefits of the clinical service outweighs the potential risks. Clinicians should discuss the service with eligible patients.
Level C: At least fair scientific evidence suggests that the clinical service provides benefits, but the balance between benefits and risks is too close for general recommendations. Clinicians need not offer it unless individual considerations apply.
Level D: At least fair scientific evidence suggests that the risks of the clinical service outweigh potential benefits. Clinicians should not routinely offer the service to asymptomatic patients.
Level I: Scientific evidence is lacking, of poor quality, or conflicting, such that the risk versus benefit balance cannot be assessed. Clinicians should help patients understand the uncertainty surrounding the clinical service.
GRADE guideline panelists may make strong or weak recommendations on the basis of further criteria. Some of the important criteria are the balance between desirable and undesirable effects (not considering cost), the quality of the evidence, values and preferences and costs (resource utilization).
Despite the differences between systems, the purposes are the same: to guide users of clinical research information on which studies are likely to be most valid. However, the individual studies still require careful critical appraisal
=== Statistical measures ===
Evidence-based medicine attempts to express clinical benefits of tests and treatments using mathematical methods. Tools used by practitioners of evidence-based medicine include:
Likelihood ratio The pre-test odds of a particular diagnosis, multiplied by the likelihood ratio, determines the post-test odds. (Odds can be calculated from, and then converted to, the [more familiar] probability.) This reflects Bayes' theorem. The differences in likelihood ratio between clinical tests can be used to prioritize clinical tests according to their usefulness in a given clinical situation.
AUC-ROC The area under the receiver operating characteristic curve (AUC-ROC) reflects the relationship between sensitivity and specificity for a given test. High-quality tests will have an AUC-ROC approaching 1, and high-quality publications about clinical tests will provide information about the AUC-ROC. Cutoff values for positive and negative tests can influence specificity and sensitivity, but they do not affect AUC-ROC.
Number needed to treat (NNT)/Number needed to harm (NNH). NNT and NNH are ways of expressing the effectiveness and safety, respectively, of interventions in a way that is clinically meaningful. NNT is the number of people who need to be treated in order to achieve the desired outcome (e.g. survival from cancer) in one patient. For example, if a treatment increases the chance of survival by 5%, then 20 people need to be treated in order for 1 additional patient to survive because of the treatment. The concept can also be applied to diagnostic tests. For example, if 1,339 women age 50–59 need to be invited for breast cancer screening over a ten-year period in order to prevent one woman from dying of breast cancer, then the NNT for being invited to breast cancer screening is 1339.
=== Quality of clinical trials ===
Evidence-based medicine attempts to objectively evaluate the quality of clinical research by critically assessing techniques reported by researchers in their publications.
Trial design considerations: High-quality studies have clearly defined eligibility criteria and have minimal missing data.
Generalizability considerations: Studies may only be applicable to narrowly defined patient populations and may not be generalizable to other clinical contexts.
Follow-up: Sufficient time for defined outcomes to occur can influence the prospective study outcomes and the statistical power of a study to detect differences between a treatment and control arm.
Power: A mathematical calculation can determine whether the number of patients is sufficient to detect a difference between treatment arms. A negative study may reflect a lack of benefit, or simply a lack of sufficient quantities of patients to detect a difference.
== Limitations and criticism ==
There are a number of limitations and criticisms of evidence-based medicine. Two widely cited categorization schemes for the various published critiques of EBM include the three-fold division of Straus and McAlister ("limitations universal to the practice of medicine, limitations unique to evidence-based medicine and misperceptions of evidence-based-medicine") and the five-point categorization of Cohen, Stavri and Hersh (EBM is a poor philosophic basis for medicine, defines evidence too narrowly, is not evidence-based, is limited in usefulness when applied to individual patients, or reduces the autonomy of the doctor/patient relationship).
In no particular order, some published objections include:

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Research produced by EBM, such as from randomized controlled trials (RCTs), may not be relevant for all treatment situations. Research tends to focus on specific populations, but individual persons can vary substantially from population norms. Because certain population segments have been historically under-researched (due to reasons such as race, gender, age, and co-morbid diseases), evidence from RCTs may not be generalizable to those populations. Thus, EBM applies to groups of people, but this should not preclude clinicians from using their personal experience in deciding how to treat each patient. One author advises that "the knowledge gained from clinical research does not directly answer the primary clinical question of what is best for the patient at hand" and suggests that evidence-based medicine should not discount the value of clinical experience. Another author stated that "the practice of evidence-based medicine means integrating individual clinical expertise with the best available external clinical evidence from systematic research."
Use of evidence-based guidelines often fits poorly for complex, multimorbid patients. This is because the guidelines are usually based on clinical studies focused on single diseases. In reality, the recommended treatments in such circumstances may interact unfavorably with each other and often lead to polypharmacy.
The theoretical ideal of EBM (that every narrow clinical question, of which hundreds of thousands can exist, would be answered by meta-analysis and systematic reviews of multiple RCTs) faces the limitation that research (especially the RCTs themselves) is expensive; thus, in reality, for the foreseeable future, the demand for EBM will always be much higher than the supply, and the best humanity can do is to triage the application of scarce resources.
Research can be influenced by biases such as political or belief bias, publication bias and conflict of interest in academic publishing. For example, studies with conflicts due to industry funding are more likely to favor their product. It has been argued that contemporary evidence based medicine is an illusion, since evidence based medicine has been corrupted by corporate interests, failed regulation, and commercialisation of academia.
Systematic Reviews methodologies are capable of bias and abuse in respect of (i) choice of inclusion criteria (ii) choice of outcome measures, comparisons and analyses (iii) the subjectivity inevitable in Risk of Bias assessments, even when codified procedures and criteria are observed. An example of all these problems can be seen in a Cochrane Review.
A lag exists between when the RCT is conducted and when its results are published.
A lag exists between when results are published and when they are properly applied.
Hypocognition (the absence of a simple, consolidated mental framework into which new information can be placed) can hinder the application of EBM.
Values: while patient values are considered in the original definition of EBM, the importance of values is not commonly emphasized in EBM training, a potential problem under current study.
A 2018 study, "Why all randomised controlled trials produce biased results", assessed the 10 most cited RCTs and argued that trials face a wide range of biases and constraints, from trials only being able to study a small set of questions amenable to randomisation and generally only being able to assess the average treatment effect of a sample, to limitations in extrapolating results to another context, among many others outlined in the study.
== Application of evidence in clinical settings ==
Despite the emphasis on evidence-based medicine, unsafe or ineffective medical practices may occur. Contributing factors include clinicians not keeping up with or acting on current evidence, the rapid pace of scientific change, financial incentives, and patient demand for tests or treatments. Even when the evidence unequivocally shows that a treatment is either not safe or ineffective, it may take many years for other treatments to be adopted.
Several factors may contribute to lack of uptake or implementation of evidence-based recommendations. These include lack of awareness at the individual clinician or patient (micro) level, lack of institutional support at the organisation level (meso) level or higher at the policy (macro) level. In other cases, significant change can require a generation of physicians to be replaced by physicians who were trained with more recent evidence.
Revision of clinical guidelines to include an implementation plan may facilitate uptake of new procedures, including analysis of the context, identifying barriers and facilitators, and designing strategies to address them.
== Education ==
Training in evidence based medicine is offered across the continuum of medical education. Educational competencies have been created for the education of health care professionals.
The Berlin questionnaire and the Fresno Test are validated instruments for assessing the effectiveness of education in evidence-based medicine. These questionnaires have been used in diverse settings.
A Campbell systematic review that included 24 trials examined the effectiveness of e-learning in improving evidence-based health care knowledge and practice. It was found that e-learning, compared to no learning, improves evidence-based health care knowledge and skills but not attitudes and behaviour. No difference in outcomes is present when comparing e-learning with face-to-face learning. Combining e-learning and face-to-face learning (blended learning) has a positive impact on evidence-based knowledge, skills, attitude and behavior. As a form of e-learning, some medical school students engage in editing Wikipedia to increase their EBM skills, and some students construct EBM materials to develop their skills in communicating medical knowledge.
== See also ==
== References ==
== Bibliography ==
== External links ==
Evidence-Based Medicine – An Oral History, JAMA and the BMJ, 2014.
Centre for Evidence-based Medicine at the University of Oxford.

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The Holocaust—the systematic killing of about six million Jews by Nazi Germany from 1941 to 1945—is the most documented genocide in history. Although there is no single document which lists the names of all Jewish victims of Nazi persecution, there is conclusive evidence that about six million Jews were murdered. There is also conclusive evidence that Jews were gassed at Auschwitz-Birkenau, the Operation Reinhard extermination camps, and in gas vans, and that there was a systematic plan by the Nazi leadership to murder them.
Evidence for the Holocaust comes in four main varieties:
Contemporary documents, including a wide variety of "letters, memos, blueprints, orders, bills, speeches"; Holocaust train schedules and statistical summaries generated by the SS; and photographs, including official photographs, clandestine photographs by survivors, aerial photographs, and film footage of the liberation of the camps. More than 3,000 tons of records were collected for the Nuremberg trials.
Later testimony from tens of thousands of eyewitnesses, including survivors such as Sonderkommandos, who directly witnessed the extermination process; perpetrators such as Nazi leaders, SS guards, and Nazi concentration camp commandants; and local townspeople. Moreover, virtually none of the perpetrators put on trial denied the reality of the systematic murder, with the most common excuse (where one was given) being that they were just following orders.
Material evidence in the form of concentration and extermination camps, which still exist with various amounts of the original structure preserved, and thousands of mass graves containing the corpses of Holocaust victims.
Circumstantial evidence: during World War II, the population of Jews in German-occupied Europe was reduced by about six million. About 2.7 million Jews were deported to Auschwitz-Birkenau, Kulmhof extermination camp, and the Operation Reinhard camps never to be seen or heard from again.
The perpetrators attempted to avoid creating explicit evidence and they also tried to destroy the documentary and material evidence of their crimes before the German defeat. Nevertheless, much of the evidence was preserved and collected by Allied investigators during and after the war, and the overwhelming evidence of the crimes ultimately made such erasure attempts futile. Collectively, the evidence refutes the arguments of Holocaust deniers that the Holocaust did not occur as described in historical scholarship.
== Hitler's involvement ==
=== Policy ===
Historians, including Ian Kershaw, Raul Hilberg, and Martin Broszat, indicate that no document exists showing that Hitler ordered the Holocaust. However, other evidence makes clear that Hitler knew about and ordered the genocide. Statements from top-ranking Nazis such as Adolf Eichmann, Joseph Goebbels, and Heinrich Himmler also indicate that Hitler orchestrated the Holocaust and statements from Hitler himself reveal his genocidal intentions toward Jewry.
=== Order and responsibility ===
In a draft of an internal memorandum, dated 18 September 1942, Reichsfuhrer SS Heinrich Himmler wrote that "in principle the Fuehrer's time is no longer to be burdened with these matters"; the memorandum goes on to outline Himmler's vision, including "The delivery of anti-social elements from the execution of their sentences to the Reich Fuehrer of SS to be worked to death. Persons under protective arrest, Jews, Gypsies, Russians and Ukrainians, Poles with more than 3-year sentences, Czechs and Germans with more than 8-year sentences according to the judgement of the Minister of Justice [Thierack]. First of all, the worst anti-social elements amongst those just mentioned are to be handed over; I shall inform the Fuhrer of this through Reichsleiter Bormann."
Nevertheless, and in contrast to the T4 euthanasia program, no document written or signed by Hitler ordering the Holocaust has ever been found. Deniers have claimed that this lack of order shows genocide was not Nazi policy.
During David Irving's unsuccessful libel action against Deborah Lipstadt, he indicated that he considered a document signed by Hitler ordering the 'Final Solution' would be the only convincing proof of Hitler's responsibility. He was, however, described as content to accuse Winston Churchill of responsibility for ordering the assassination of General Sikorski, despite having no documentary evidence to support his claim. Mr Justice Gray concluded that this was a double standard.
Historians have documented evidence that as Germany's defeat became imminent and the Nazi leaders realized that they would most likely be captured and brought to trial, a great effort to destroy all of the evidence of mass extermination was made. In the spring of 1942, Himmler ordered all of the traces of murdered Russian Jews and all of the traces of murdered prisoners of war to be removed from the occupied territories of the Soviet Union. As one of many examples, the bodies of the 25,000 mostly Latvian Jews whom Friedrich Jeckeln and the soldiers under his command had shot at Rumbula (near Riga) in late 1941 were dug up and burned in 1943.
In mid-1942, Reinhard Heydrich, through Heinrich Mueller, Chief of the Gestapo, ordered Paul Blobel in Sonderaktion 1005 to remove all traces of the mass executions in the East carried out by the Einsatzgruppen. After Blobel and his staff developed a special incineration process, destruction of evidence at Belzec and Sobibor followed in late 1942. In February 1943, Himmler personally visited Treblinka and ordered the commandants to destroy records, crematoria, and other signs of mass extermination.
In the Posen speeches of October 1943, Himmler explicitly referred to the extermination of the Jews of Europe and further stated that the genocide must be permanently kept secret. On 4 October, he said:

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I also want to refer here very frankly to a very difficult matter. We can now very openly talk about this among ourselves, and yet we will never discuss this publicly. Just as we did not hesitate on June 30, 1934, to perform our duty as ordered and put comrades who had failed up against the wall and execute them, we also never spoke about it, nor will we ever speak about it. Let us thank God that we had within us enough self-evident fortitude never to discuss it among us, and we never talked about it. Every one of us was horrified, and yet every one clearly understood that we would do it next time, when the order is given and when it becomes necessary.
I am now referring to the evacuation of the Jews, to the extermination of the Jewish people.
Historian Peter Longerich states that Hitler "avoided giving a clear written order to exterminate Jewish civilians". Wide protest was evoked when Hitler's authorisation of the T4 program became public knowledge in Germany, and he was forced to put a halt to it as a result (nonetheless it continued discreetly). This made Hitler realise that such undertakings must be done secretly in order to avoid criticism. Critics also point out that if Hitler did sign such an order in the first place, it would have been one of the first documents to be destroyed.
Evidence of a verbal order from Hitler includes a handwritten note by Himmler on a meeting with Hitler at the Wolfsschanze on 18 December 1941, which read: "Jewish Question; to be exterminated as partisans". Historians have argued that this indicates Hitler gave a verbal order to Himmler at this meeting for the Einsatzgruppen to target Jews under the guise of anti-partisan warfare.
According to Felix Kersten's memoirs, Himmler told him that the extermination of the Jews was expressly ordered by Hitler and had been delegated to Himmler.
==== According to Nazis ====
Many statements from the Nazis from 1941 onwards addressed the imminent extermination of the Jews.
In a draft of an internal memorandum, dated 25 October 1941, Heinrich Himmler wrote:
As the affairs now stand, there are no objections against doing away with those Jews who are not able to work, with the Brack remedy.
Joseph Goebbels had frequent discussions with Hitler about the fate of the Jews, a subject which they discussed almost every time they met, and frequently wrote about it in his personal diary. In his personal diary he wrote:
14 February 1942: "The Führer once again expressed his determination to clean up the Jews in Europe pitilessly. There must be no squeamish sentimentalism about it. The Jews have deserved the catastrophe that has now overtaken them. Their destruction will go hand in hand with the destruction of our enemies. We must hasten this process with cold ruthlessness."
27 March 1942: "A judgment is being visited upon the Jews that, while barbaric, is fully deserved by them. The prophecy which the Führer made about them for having brought on a new world war is beginning to come true in a most terrible manner. One must not be sentimental in these matters. If we did not fight the Jews, they would destroy us. It's a life-and-death struggle between the Aryan race and the Jewish bacillus."
On 16 November 1941, Goebbels published an article "The Jews are to blame" which returned to Hitler's prophecy of 1939 and stated that world Jewry was suffering a "gradual process of extermination". Goebbels wrote: "Some six million Jews still live in the East, and this question can only be solved by a biological extermination of the whole of Jewry in Europe".
On 13 March 1945, Goebbels wrote in his diary that the "rest of the world" should follow Germany's example in "destroying the Jews", he wrote also about how the Jews in Germany at that point had been almost totally destroyed. This diary contains numerous other references to the mass extermination of Jews, including how "tens of thousands of them are liquidated" in eastern occupied territory, and that "the greater the number of Jews liquidated, the more consolidated will the situation in Europe be after this war." When speaking about this document under oath, David Irving is quoted as saying "There is no explicit reference...to the liquidation of Jews" and critics of Holocaust denial consequently state that it is dishonest to say such a thing when it is entirely contradicted by the diary of one of Hitler's closest associates.
When questioned by interrogators if orders for the extermination of Jews were delegated in writing by Himmler, Adolf Eichmann states:

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I never saw a written order, Herr Hauptmann. All I know is that Heydrich said to me: "The Führer has ordered the physical extermination of the Jews." He said that as clearly and surely as I'm repeating it now.
Critics state that Eichmann gives a virtually identical account of this in his memoirs, and state that it is also asserted that Eichmann never even asked for a written order, on the basis that "Hitler's wish as expressed through Himmler and Heydrich was good enough for him". Eichmann's memoirs were recorded by Willem Sassen before he was captured, and Eichmann's lawyer tried to prevent them from being presented as evidence to avoid any detriment against his case.
In a speech, David Irving states that Heydrich told Eichmann, "The Führer has given the order for the physical destruction of the Jews". Irving admits that this contradicts his view that "Hitler wasn't involved", but explains it by suggesting that a completely different meaning can be construed, i.e. "the extirpation of Judaism" as opposed to the physical destruction of Jews if one changes "just one or two words". Critics of this view state that historians should not change words if their documents contradict their claims, and consequently point out five instances where Eichmann unambiguously states "physical extermination" during his interrogation.
At a conference in 1941 discussing the Jewish Question, Alfred Rosenberg said:
Some six million Jews still live in the East, and this question can only be solved by a biological extermination of the whole of Jewry in Europe. The Jewish Question will only be solved for Germany when the last Jew has left German territory, and for Europe when not a single Jew stands on the European continent as far as the Urals... And to this end it is necessary to force them beyond the Urals or otherwise bring about their eradication.
At the Einsatzgruppen Trial in 1947, SS-Obersturmbannfuhrer Martin Sandberger recalled that his superior, SS-Gruppenfuhrer Bruno Streckenbach, had informed him and other Einsatzgruppen commanders of an order from Hitler to eliminate all Jews in the Eastern Territories at a meeting at the Palais Prinz Albrecht in 1941.
Rudolf Höss, commandant of the Auschwitz concentration camp, wrote a series of memoirs about his role in the Holocaust while awaiting execution after the war. In these memoirs Höss stated that Himmler had briefed him about the Final Solution and his role in it in summer 1941; during the meeting, Himmler told him that the order for the Final Solution came directly from Hitler.
=== Awareness ===
Congruent with the evidence that shows Hitler was responsible for the order to murder Jews, there is also evidence that shows he was made aware of the process. Gestapo Chief Heinrich Müller sent a telegram on 2 August 1941, ordering that "especially interesting illustrative" material should be sent to Berlin because, "the Führer should be presented with continuous reports on the work of Einsatzgruppen in the East from here". At the end of December 1942 Hitler received a document from Himmler entitled, "Report to the Führer on Combating Partisans", stating that 363,211 Jews had been murdered by the Einsatzgruppen in August–November 1942. This document was specifically printed in large font that Hitler could read without glasses and was marked "Shown to the Führer".
== Himmler's speeches ==
Critics of Holocaust denial state that the claim by deniers of no Nazi plan to exterminate the Jews is discredited by Himmler in a speech made on 4 October 1943 to a gathering of SS officers in Poznań, where he said:
In a speech at Sonthofen on 24 May 1944, Himmler said to a group of German generals:
I believe, gentlemen, that you know me well enough to realize that I am not a bloodthirsty man nor a man who takes pleasure or finds sport in the harsher things he must do. On the other hand, I have strong nerves and a great sense of duty—if I do say so myself—and when I recognize the necessity to do something, I will do it unflinchingly. As to the Jewish women and children, I did not believe I had a right to let these children grow up to become avengers who would kill our fathers [sic] and grandchildren. That, I thought, would be cowardly.
== Use of gas chambers ==
The German firm Topf and Sons manufactured gas chambers to be used in concentration camps for extermination.
Despite the difficulty of finding traces of this material, in February 1990, Professor Jan Markiewicz, Director of the Institute of Forensic Research in Kraków, redid the analysis. Markiewicz and his team used microdiffusion techniques to test for cyanide in samples from the suspected gas chambers, from delousing chambers, and from control areas elsewhere within Auschwitz. The control samples tested negative, while cyanide residue was found in high concentrations in the delousing chambers, and lower concentrations in the homicidal gas chambers. This is consistent with the amounts required to kill lice and humans.
The search for cyanide in the bricks of buildings said to have been gas chambers was important, because the pesticide Zyklon B would generate such a residue. This was the gas most often cited as the murder instrument for prisoners in the gas chambers, supported by both testimony and evidence collected of Nazi policy.
Another claim made by Holocaust deniers is that there were no specially-constructed vents in the gas chambers through which Zyklon B could be released. The BBC offers a response showing that this requires disregard of much documentation:

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Deniers have said for years that physical evidence is lacking because they have seen no holes in the roof of the Birkenau gas chamber where the Zyklon was poured in. (In some of the gas chambers the Zyklon B was poured in through the roof, while in others it was thrown in through the windows.) The roof was dynamited at war's end, and today lies broken in pieces, but three of the four original holes were positively identified in a recent paper. Their location in the concrete matches with eyewitness testimony, aerial photos from 1944, and a ground photo from 1943. The physical evidence shows unmistakably that the Zyklon holes were cast into the concrete when the building was constructed.
Deniers also claim that the doors of gas chambers, some of which were made out of wood, were not airtight enough for the chambers to have worked correctly, assertion that has been thrououghly debunked.
Cremation in the open at the Reinhard extermination camps (Treblinka, Sobibor and Belzec) was discussed at Nuremberg on 7 April 1946 by Georg Konrad Morgen, SS judge and lawyer who investigated crimes committed in Nazi concentration camps. He stated: "The whole thing was like an assembly line. At the last stop they reached a big room, and were told that this was the bath. When the last one was in, the doors were shut and the gas was let into the room. As soon as death taken place in (sic), the ventilators were started. When the air was breathable, the doors were opened, and the Jewish workers removed the bodies. By means of a special process which Wirth had invented, they were burned in the open air without the use of fuel."
There is well-documented evidence that other ash was used as fertilizer in nearby fields. Photographs of Treblinka taken by the camp commandant show what looks to be ash piles being distributed by steam shovels.
The Nizkor Project and other sources have stated that the minimal concentration of Zyklon B to be explosive is 56,000 parts per million, while 300 parts per million is fatal to humans, as is evidenced in The Merck Index and the CRC Handbook of Chemistry and Physics. In fact, the Nazis' own documentation stated "Danger of explosion: 75 grams of HCN in 1 cubic meter of air. Normal application approx. 8–10 grams per cubic meter, therefore not explosive."
The Institute for Historical Review publicly offered a reward of $50,000 for verifiable "proof that gas chambers for the purpose of killing human beings existed at or in Auschwitz." Mel Mermelstein, a survivor of Auschwitz, submitted his own testimony as proof but it was ignored. He then sued IHR in the United States and the case was subsequently settled for $50,000, plus $40,000 in damages for personal suffering. The court declared the statement that "Jews were gassed to death at the Auschwitz Concentration Camp in Poland during the summer of 1944" was a fact.
== Victims ==
=== Six million ===
The vast majority of scholars, institutions, and one Nazi official estimate between five and six million Jews perished during the Holocaust. With approximately 4.5 million Jewish victims' names collected by Yad Vashem, numerous documents and archives discovered after the war gave meticulous accounts of the exterminations that took place at the death camps (such as Auschwitz and Treblinka).
=== Jewish population ===
The 1932 American Jewish Yearbook estimates the total number of Jews in the world at 15,192,218, of whom 9,418,248 resided in Europe. However, the 1947 yearbook states: "Estimates of the world Jewish population have been assembled by the American Jewish Joint Distribution Committee (except for the United States and Canada) and are probably the most authentic available at the present time. The figures reveal that the total Jewish population of the world has decreased by one-third from about 16,600,000 in 1939 to about 11,000,000 in 1946 as the result of the annihilation by the Nazis of more than five and a half million European Jews. In Europe only an estimated 3,642,000 remain of the total Jewish pre-war population of approximately 9,740,000." These numbers are also consistent with the findings of the Anglo-American Committee of Inquiry, Appendix III, in 1946.
== Nazi documentation ==
The Nazis used figures of between 9 and 11 million for the Jewish population of Europe, as evidenced in the notes of the Wannsee Conference. In fact, the Nazis methodically recorded the ongoing reduction of the Jewish population, as in the Korherr Report, which gave the status of the Final Solution through December 1942. The Höfle Telegram was sent by Hermann Höfle on 11 January 1943 to Adolf Eichmann in Berlin and detailed the number of Jews murdered in the concentration camps. In the year 1942 alone, the telegram lists 1,274,166 Jews were exterminated in the four camps of Aktion Reinhard.
The Korherr Report, compiled by an SS statistician, gave a conservative total of 2,454,000 Jews deported to extermination camps or murdered by the Einsatzgruppen. The complete status reports of the Einsatzgruppen death squads were found in the archives of the Gestapo when it was searched by the U.S. Army, and the accuracy attested to by the former Einsatzgruppen members who testified during war crime trials and at other times. These reports alone list an additional 1,500,000 or so murders during mass shootings, the vast majority of these victims were Jews. Further, surviving Nazi documentation spells out their plans to murder the Jews of Europe (see the Wannsee Conference), recorded the trains arriving at various death camps, and included photographs and films of many atrocities.

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== Testimonies ==
There is a voluminous amount of testimony from tens of thousands of survivors of the Holocaust, as well as the testimony of captured Nazi officers at the Nuremberg Trials and other times. Höss's testimony did not consist of merely a signed confession; while in jail he also wrote two volumes of memoirs and gave extensive testimony outside of the Nuremberg proceedings. Further, his testimony agrees with that of other contemporary written accounts by Auschwitz officials, such as Pery Broad, an SS man stationed at Auschwitz while Höss was the commandant, and the diary kept by SS physician at Auschwitz Johann Kremer, as well as the testimony of hundreds of camp guards and victims. Auschwitz guard Reinhold Hanning even testified that it was common knowledge among camp personnel that "the majority of people who arrived in the trains were killed". In addition, former SS personnel have criticised Holocaust denial. SS-Oberscharführer Josef Klehr said that anyone who maintains that nobody was gassed at Auschwitz must be "crazy or on the wrong". SS-Unterscharführer Oswald Kaduk stated that he did not consider those who maintain such a thing as normal people. Karl Frenzel, a senior officer at the Sobibor extermination camp, stated in a 1983 interview that "It is wrong to say that it never happened" in reference to Jews being gassed at the camp. Hearing about Holocaust denial compelled former SS-Rottenführer Oskar Gröning to publicly speak about what he witnessed at Auschwitz, and denounce Holocaust deniers, stating:
I would like you to believe me. I saw the gas chambers. I saw the crematoria. I saw the open fires. I was on the ramp when the selections took place. I would like you to believe that these atrocities happened because I was there.
Hans Münch, a former SS physician, signed a document certifying what he witnessed at Auschwitz: "thousands of people gassed", and the usage of Zyklon B in gas chambers. According to Münch's estimation, prisoners died within three to five minutes of exposure to Zyklon B. In an interview on Swedish television in 1981 Münch described the extermination process in detail and confirmed that "special treatment" in the context of Auschwitz referred to physical extermination.
During Fedorenko v. United States, a deportation case involving former Treblinka guard Feodor Fedorenko, he testified that he had been stationed in a guard tower overlooking the camp and admitted that the gas chambers were visible from this vantage point and that he had witnessed dead bodies being removed from the gas chambers on multiple occasions.
In the 1983 Holocaust documentary Shoah, Unterscharführer Franz Suchomel, tricked into an interview with false promises of anonymity, described his time at the Treblinka extermination camp. Suchomel related to the interviewer, Claude Lanzmann, how he saw dead bodies being removed from the gas chambers during a tour of the camp before explaining in depth the extermination of Jews at the camp through both gassing and shooting.
Sonderkommandos provide another key piece of testimony. These were Jewish prisoners who helped march Jews to the gas chambers, and later dragged the bodies to the crematoria. Since they witnessed the entire process, their testimony is vital in confirming that the gas chambers were used for murderous purposes and the scale to which they were used.
Other key testimony comes from non-Jewish survivors of the camps such as Catholic French Resistance member André Rogerie who was held in seven different camps, and who as a member of the Resistance was not targeted for extermination but for hard labor and survived. After the war Rogerie wrote and testified extensively about his experiences in the camps including Auschwitz-Birkenau, where he viewed and produced the oldest contemporary sketch of a camp crematorium.
== References ==
=== Citations ===
=== Sources ===

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The infinitesimally low concentration of homeopathic preparations, which often lack even a single molecule of the diluted substance, has been the basis of questions about the effects of the preparations since the 19th century. Modern advocates of homeopathy have proposed a concept of "water memory", according to which water "remembers" the substances mixed in it, and transmits the effect of those substances when consumed. This concept is inconsistent with the current understanding of matter, and water memory has never been demonstrated to exist, in terms of any detectable effect, biological or otherwise.
James Randi and the 10:23 campaign groups have highlighted the lack of active ingredients in most homeopathic products by taking large 'overdoses'. None of the hundreds of demonstrators in the UK, Australia, New Zealand, Canada and the US were injured and "no one was cured of anything, either".
Outside of the alternative medicine community, scientists have long considered homeopathy a sham or a pseudoscience, and the mainstream medical community regards it as quackery. There is an overall absence of sound statistical evidence of therapeutic efficacy, which is consistent with the lack of any biologically plausible pharmacological agent or mechanism.
Abstract concepts within theoretical physics have been invoked to suggest explanations of how or why preparations might work, including quantum entanglement, quantum nonlocality, the theory of relativity and chaos theory. Contrariwise, quantum superposition has been invoked to explain why homeopathy does not work in double-blind trials. However, the explanations are offered by nonspecialists within the field, and often include speculations that are incorrect in their application of the concepts and not supported by actual experiments. Several of the key concepts of homeopathy conflict with fundamental concepts of physics and chemistry. The use of quantum entanglement to explain homeopathy's purported effects is "patent nonsense", as entanglement is a delicate state that rarely lasts longer than a fraction of a second. While entanglement may result in certain aspects of individual subatomic particles acquiring linked quantum states, this does not mean the particles will mirror or duplicate each other, nor cause health-improving transformations.
== Plausibility ==
The proposed mechanisms for homeopathy are precluded from having any effect by the laws of physics and physical chemistry. The extreme dilutions used in homeopathic preparations usually leave not one molecule of the original substance in the final product.
A number of speculative mechanisms have been advanced to counter this, the most widely discussed being water memory, though this is now considered erroneous since short-range order in water only persists for about 1 picosecond. No evidence of stable clusters of water molecules was found when homeopathic preparations were studied using nuclear magnetic resonance, and many other physical experiments in homeopathy have been found to be of low methodological quality, which precludes any meaningful conclusion. Existence of a pharmacological effect in the absence of any true active ingredient is inconsistent with the law of mass action and the observed dose-response relationships characteristic of therapeutic drugs (whereas placebo effects are non-specific and unrelated to pharmacological activity).
Homeopaths contend that their methods produce a therapeutically active preparation, selectively including only the intended substance, though critics note that any water will have been in contact with millions of different substances throughout its history, and homeopaths have not been able to account for a reason why only the selected homeopathic substance would be a special case in their process. For comparison, ISO 3696:1987 defines a standard for water used in laboratory analysis; this allows for a contaminant level of ten parts per billion, 4C in homeopathic notation. This water may not be kept in glass as contaminants will leach out into the water.
Practitioners of homeopathy hold that higher dilutions―described as being of higher potency―produce stronger medicinal effects. This idea is also inconsistent with observed dose-response relationships, where effects are dependent on the concentration of the active ingredient in the body. This dose-response relationship has been confirmed in myriad experiments on organisms as diverse as nematodes, rats, and humans. Some homeopaths contend that the phenomenon of hormesis may support the idea of dilution increasing potency, but the dose-response relationship outside the zone of hormesis declines with dilution as normal, and nonlinear pharmacological effects do not provide any credible support for homeopathy.
Physicist Robert L. Park, former executive director of the American Physical Society, is quoted as saying: "since the least amount of a substance in a solution is one molecule, a 30C solution would have to have at least one molecule of the original substance dissolved in a minimum of 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 [or 1060] molecules of water. This would require a container more than 30,000,000,000 times the size of the Earth." Park is also quoted as saying that, "to expect to get even one molecule of the 'medicinal' substance allegedly present in 30X pills, it would be necessary to take some two billion of them, which would total about a thousand tons of lactose plus whatever impurities the lactose contained".
The laws of chemistry state that there is a limit to the dilution that can be made without losing the original substance altogether. This limit, which is related to the Avogadro constant, is roughly equal to homeopathic dilutions of 12C or 24X (1 part in 1024).
Scientific tests run by both the BBC's Horizon and ABC's 20/20 programmes were unable to differentiate homeopathic dilutions from water, even when using tests suggested by homeopaths themselves.
In May 2018, the German skeptical organization GWUP issued an invitation to individuals and groups to respond to its challenge "to identify homeopathic preparations in high potency and to give a detailed description on how this can be achieved reproducibly." The first participant to correctly identify selected homeopathic preparations under an agreed-upon protocol will receive €50,000.
== Efficacy ==

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No individual homeopathic preparation has been unambiguously shown by research to be different from placebo. The methodological quality of the primary research was generally low, with such problems as weaknesses in study design and reporting, small sample size, and selection bias. Since better quality trials have become available, the evidence for efficacy of homeopathy preparations has diminished; the highest-quality trials indicate that the preparations themselves exert no intrinsic effect. A review conducted in 2010 of all the pertinent studies of "best evidence" produced by the Cochrane Collaboration concluded that "the most reliable evidence – that produced by Cochrane reviews – fails to demonstrate that homeopathic medicines have effects beyond placebo."
=== Government level reviews ===
Government-level reviews have been conducted in recent years by Switzerland (2005), the United Kingdom (2009), Australia (2015) and the European Academies' Science Advisory Council (2017).
The Swiss programme for the evaluation of complementary medicine (PEK) resulted in the peer-reviewed Shang publication (see Systematic reviews and meta-analyses of efficacy) and a controversial competing analysis by homeopaths and advocates led by Gudrun Bornhöft and Peter Matthiessen, which has misleadingly been presented as a Swiss government report by homeopathy proponents, a claim that has been repudiated by the Swiss Federal Office of Public Health. The Swiss Government terminated reimbursement, though it was subsequently reinstated after a political campaign and referendum for a further six-year trial period.
The United Kingdom's House of Commons Science and Technology Committee sought written evidence and submissions from concerned parties and, following a review of all submissions, concluded that there was no compelling evidence of effect other than placebo and recommended that the Medicines and Healthcare products Regulatory Agency (MHRA) should not allow homeopathic product labels to make medical claims, that homeopathic products should no longer be licensed by the MHRA, as they are not medicines, and that further clinical trials of homeopathy could not be justified. They recommended that funding of homeopathic hospitals should not continue, and NHS doctors should not refer patients to homeopaths. By February 2011 only one-third of primary care trusts still funded homeopathy and by 2012 no British universities offered homeopathy courses. In July 2017, as part of a plan to save £200m a year by preventing the "misuse of scarce" funding, the NHS announced that it would no longer provide homeopathic medicines. A legal appeal by the British Homeopathic Association against the decision was rejected in June 2018.
The Australian National Health and Medical Research Council completed a comprehensive review of the effectiveness of homeopathic preparations in 2015, in which it concluded that "there were no health conditions for which there was reliable evidence that homeopathy was effective. No good-quality, well-designed studies with enough participants for a meaningful result reported either that homeopathy caused greater health improvements than placebo, or caused health improvements equal to those of another treatment."
On September 20, 2017, the European Academies' Science Advisory Council (EASAC) published its official analysis and conclusion on the use of homeopathic products, finding a lack of evidence that homeopathic products are effective, and raising concerns about quality control.
=== Publication bias and other methodological problems ===
The fact that individual randomized controlled trials have given positive results is not in contradiction with an overall lack of statistical evidence of efficacy. A small proportion of randomized controlled trials inevitably provide false-positive outcomes due to the play of chance: a statistically significant positive outcome is commonly adjudicated when the probability of it being due to chance rather than a real effect is no more than 5%―a level at which about 1 in 20 tests can be expected to show a positive result in the absence of any therapeutic effect. Furthermore, trials of low methodological quality (i.e. ones that have been inappropriately designed, conducted or reported) are prone to give misleading results. In a systematic review of the methodological quality of randomized trials in three branches of alternative medicine, Linde et al. highlighted major weaknesses in the homeopathy sector, including poor randomization. A separate 2001 systematic review that assessed the quality of clinical trials of homeopathy found that such trials were generally of lower quality than trials of conventional medicine.
A related issue is publication bias: researchers are more likely to submit trials that report a positive finding for publication, and journals prefer to publish positive results. Publication bias has been particularly marked in alternative medicine journals, where few of the published articles (just 5% during the year 2000) tend to report null results. Regarding the way in which homeopathy is represented in the medical literature, a systematic review found signs of bias in the publications of clinical trials (towards negative representation in mainstream medical journals, and vice versa in alternative medicine journals), but not in reviews.
Positive results are much more likely to be false if the prior probability of the claim under test is low.
=== Systematic reviews and meta-analyses of efficacy ===
Both meta-analyses, which statistically combine the results of several randomized controlled trials, and other systematic reviews of the literature are essential tools to summarize evidence of therapeutic efficacy. Early systematic reviews and meta-analyses of trials evaluating the efficacy of homeopathic preparations in comparison with placebo more often tended to generate positive results, but appeared unconvincing overall. In particular, reports of three large meta-analyses warned readers that firm conclusions could not be reached, largely due to methodological flaws in the primary studies and the difficulty in controlling for publication bias. The positive finding of one of the most prominent of the early meta-analyses, published in The Lancet in 1997 by Linde et al., was later reframed by the same research team, who wrote:

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The evidence of bias [in the primary studies] weakens the findings of our original meta-analysis. Since we completed our literature search in 1995, a considerable number of new homeopathy trials have been published. The fact that a number of the new high-quality trials ... have negative results, and a recent update of our review for the most "original" subtype of homeopathy (classical or individualized homeopathy), seem to confirm the finding that more rigorous trials have less-promising results. It seems, therefore, likely that our meta-analysis at least overestimated the effects of homeopathic treatments.
Subsequent work by John Ioannidis and others has shown that for treatments with no prior plausibility, the chances of a positive result being a false positive are much higher, and that any result not consistent with the null hypothesis should be assumed to be a false positive.
A systematic review of the available systematic reviews confirmed in 2002 that higher-quality trials tended to have less positive results, and found no convincing evidence that any homeopathic preparation exerts clinical effects different from placebo.
In 2005, The Lancet medical journal published a meta-analysis of 110 placebo-controlled homeopathy trials and 110 matched medical trials based upon the Swiss government's Programme for Evaluating Complementary Medicine, or PEK. The study concluded that its findings were "compatible with the notion that the clinical effects of homeopathy are placebo effects". This was accompanied by an editorial pronouncing "The end of homoeopathy". A 2017 systematic review and meta-analysis found that the most reliable evidence did not support the effectiveness of non-individualized homeopathy. The authors noted that "the quality of the body of evidence is low."
Other meta-analyses include homeopathic treatments to reduce cancer therapy side-effects following radiotherapy and chemotherapy, allergic rhinitis, attention-deficit hyperactivity disorder and childhood diarrhoea, adenoid vegetation, asthma, upper respiratory tract infection in children, insomnia, fibromyalgia, psychiatric conditions and Cochrane Library systematic reviews of homeopathic treatments for asthma, dementia, attention-deficit hyperactivity disorder, induction of labour, upper respiratory tract infections in children, and irritable bowel syndrome. Other reviews covered osteoarthritis, migraines, postoperative ecchymosis and edema, delayed-onset muscle soreness, preventing postpartum haemorrhage, or eczema and other dermatological conditions.
Some clinical trials have tested individualized homeopathy, and there have been reviews of this, specifically. A 1998 review found 32 trials that met their inclusion criteria, 19 of which were placebo-controlled and provided enough data for meta-analysis. These 19 studies showed a pooled odds ratio of 1.17 to 2.23 in favour of individualized homeopathy over the placebo, but no difference was seen when the analysis was restricted to the methodologically best trials. The authors concluded that "the results of the available randomized trials suggest that individualized homeopathy has an effect over placebo. The evidence, however, is not convincing because of methodological shortcomings and inconsistencies." Jay Shelton, author of a book on homeopathy, has stated that the claim assumes without evidence that classical, individualized homeopathy works better than nonclassical variations. A 2014 systematic review and meta-analysis found that individualized homeopathic remedies may be slightly more effective than placebos, though the authors noted that their findings were based on low- or unclear-quality evidence. The same research team later reported that taking into account model validity did not significantly affect this conclusion.
The results of reviews are generally negative or only weakly positive, and reviewers consistently report the poor quality of trials. The finding of Linde et al. that more rigorous studies produce less positive results is supported in several and contradicted by none.
=== Statements by major medical organizations ===
Health organizations such as the UK's National Health Service, the American Medical Association, the FASEB, and the National Health and Medical Research Council of Australia, have issued statements of their conclusion that there is "no good-quality evidence that homeopathy is effective as a treatment for any health condition". In 2009, World Health Organization official Mario Raviglione criticized the use of homeopathy to treat tuberculosis; similarly, another WHO spokesperson argued there was no evidence homeopathy would be an effective treatment for diarrhoea. They warned against the use of homeopathy for serious conditions such as depression, HIV and malaria.
The American College of Medical Toxicology and the American Academy of Clinical Toxicology recommend that no one use homeopathic treatment for disease or as a preventive health measure. These organizations report that no evidence exists that homeopathic treatment is effective, but that there is evidence that using these treatments produces harm and can bring indirect health risks by delaying conventional treatment.
== Explanations of perceived effects ==
Science offers a variety of explanations for how homeopathy may appear to cure diseases or alleviate symptoms even though the preparations themselves are inert:
The placebo effect – the intensive consultation process and expectations for the homeopathic preparations may cause the effect.
Therapeutic effect of the consultation – the care, concern, and reassurance a patient experiences when opening up to a compassionate caregiver can have a positive effect on the patient's well-being.
Unassisted natural healing – time and the body's ability to heal without assistance can eliminate many diseases of their own accord.
Unrecognized treatments – an unrelated food, exercise, environmental agent, or treatment for a different ailment, may have occurred.
Regression towards the mean – since many diseases or conditions are cyclical, symptoms vary over time and patients tend to seek care when discomfort is greatest; they may feel better anyway but because of the timing of the visit to the homeopath they attribute improvement to the preparation taken.
Non-homeopathic treatment – patients may also receive standard medical care at the same time as homeopathic treatment, and the former is responsible for improvement.
Cessation of unpleasant treatment – often homeopaths recommend patients stop getting medical treatment such as surgery or drugs, which can cause unpleasant side-effects; improvements are attributed to homeopathy when the actual cause is the cessation of the treatment causing side-effects in the first place, but the underlying disease remains untreated and still dangerous to the patient.
== Purported effects in other biological systems ==

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While some articles have suggested that homeopathic solutions of high dilution can have statistically significant effects on organic processes including the growth of grain, histamine release by leukocytes, and enzyme reactions, such evidence is disputed since attempts to replicate them have failed. A 2007 systematic review of high-dilution experiments found that none of the experiments with positive results could be reproduced by all investigators.
In 1987, French immunologist Jacques Benveniste submitted a paper to the journal Nature while working at INSERM. The paper purported to have discovered that basophils, a type of white blood cell, released histamine when exposed to a homeopathic dilution of anti-immunoglobulin E antibody. The journal editors, sceptical of the results, requested that the study be replicated in a separate laboratory. Upon replication in four separate laboratories the study was published. Still sceptical of the findings, Nature assembled an independent investigative team to determine the accuracy of the research, consisting of Nature editor and physicist Sir John Maddox, American scientific fraud investigator and chemist Walter Stewart, and sceptic James Randi. After investigating the findings and methodology of the experiment, the team found that the experiments were "statistically ill-controlled", "interpretation has been clouded by the exclusion of measurements in conflict with the claim", and concluded, "We believe that experimental data have been uncritically assessed and their imperfections inadequately reported." James Randi stated that he doubted that there had been any conscious fraud, but that the researchers had allowed "wishful thinking" to influence their interpretation of the data.
In 2001 and 2004, Madeleine Ennis published a number of studies that reported that homeopathic dilutions of histamine exerted an effect on the activity of basophils. In response to the first of these studies, Horizon aired a programme in which British scientists attempted to replicate Ennis' results; they were unable to do so.
== Ethics and safety ==
The provision of homeopathic preparations has been described as unethical. Michael Baum, Professor Emeritus of Surgery and visiting Professor of Medical Humanities at University College London (UCL), has described homoeopathy as a "cruel deception".
Edzard Ernst, the first professor of complementary medicine in the United Kingdom and a former homeopathic practitioner, has expressed his concerns about pharmacists who violate their ethical code by failing to provide customers with "necessary and relevant information" about the true nature of the homeopathic products they advertise and sell:
"My plea is simply for honesty. Let people buy what they want, but tell them the truth about what they are buying. These treatments are biologically implausible and the clinical tests have shown they don't do anything at all in human beings. The argument that this information is not relevant or important for customers is quite simply ridiculous."
Patients who choose to use homeopathy rather than evidence-based medicine risk missing timely diagnosis and effective treatment of serious conditions such as cancer.
In 2013 the UK Advertising Standards Authority concluded that the Society of Homeopaths were targeting vulnerable ill people and discouraging the use of essential medical treatment while making misleading claims of efficacy for homeopathic products.
In 2015 the Federal Court of Australia imposed penalties on a homeopathic company, Homeopathy Plus! Pty Ltd and its director, for making false or misleading statements about the efficacy of the whooping cough vaccine and homeopathic remedies as an alternative to the whooping cough vaccine, in breach of the Australian Consumer Law.

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=== Adverse effects ===
Some homeopathic preparations involve poisons such as Belladonna, arsenic, and poison ivy, which are highly diluted in the homeopathic preparation. In rare cases, the original ingredients are present at detectable levels. This may be due to improper preparation or intentional low dilution. Serious adverse effects such as seizures and death have been reported or associated with some homeopathic preparations.
On September 30, 2016, the FDA issued a safety alert to consumers warning against the use of homeopathic teething gels and tablets following reports of adverse events after their use. The agency recommended that parents discard these products and "seek advice from their health care professional for safe alternatives" to homeopathy for teething. The pharmacy CVS announced, also on September 30, that it was voluntarily withdrawing the products from sale and on October 11 Hyland's (the manufacturer) announced that it was discontinuing their teething medicine in the United States though the products remain on sale in Canada. On October 12, Buzzfeed reported that the regulator had "examined more than 400 reports of seizures, fever and vomiting, as well as 10 deaths" over a six-year period. The investigation (including analyses of the products) is still ongoing and the FDA does not know yet if the deaths and illnesses were caused by the products. However a previous FDA investigation in 2010, following adverse effects reported then, found that these same products were improperly diluted and contained "unsafe levels of belladonna, also known as deadly nightshade" and that the reports of serious adverse events in children using this product were "consistent with belladonna toxicity".
Instances of arsenic poisoning have occurred after use of arsenic-containing homeopathic preparations. Zicam Cold remedy Nasal Gel, which contains 2X (1:100) zinc gluconate, reportedly caused a small percentage of users to lose their sense of smell; 340 cases were settled out of court in 2006 for US$12 million. In 2009, the FDA advised consumers to stop using three discontinued cold remedy Zicam products because it could cause permanent damage to users' sense of smell. Zicam was launched without a New Drug Application (NDA) under a provision in the FDA's Compliance Policy Guide called "Conditions under which homeopathic drugs may be marketed" (CPG 7132.15), but the FDA warned Matrixx Initiatives, its manufacturer, via a Warning Letter that this policy does not apply when there is a health risk to consumers.
A 2000 review by homeopaths reported that homeopathic preparations are "unlikely to provoke severe adverse reactions". In 2012, a systematic review evaluating evidence of homeopathy's possible adverse effects concluded that "homeopathy has the potential to harm patients and consumers in both direct and indirect ways". One of the reviewers, Edzard Ernst, supplemented the article on his blog, writing: "I have said it often and I say it again: if used as an alternative to an effective cure, even the most 'harmless' treatment can become life-threatening." A 2016 systematic review and meta-analysis found that, in homeopathic clinical trials, adverse effects were reported among the patients who received homeopathy about as often as they were reported among patients who received placebo or conventional medicine.
=== Lack of efficacy ===
The lack of convincing scientific evidence supporting its efficacy and its use of preparations without active ingredients have led to characterizations as pseudoscience and quackery, or, in the words of a 1998 medical review, "placebo therapy at best and quackery at worst". The Russian Academy of Sciences considers homeopathy a "dangerous 'pseudoscience' that does not work", and "urges people to treat homeopathy 'on a par with magic'". The Chief Medical Officer for England, Dame Sally Davies, has stated that homeopathic preparations are "rubbish" and do not serve as anything more than placebos. Jack Killen, acting deputy director of the National Center for Complementary and Alternative Medicine, says homeopathy "goes beyond current understanding of chemistry and physics". He adds: "There is, to my knowledge, no condition for which homeopathy has been proven to be an effective treatment." Ben Goldacre says that homeopaths who misrepresent scientific evidence to a scientifically illiterate public, have "... walled themselves off from academic medicine, and critique has been all too often met with avoidance rather than argument". Homeopaths often prefer to ignore meta-analyses in favour of cherry picked positive results, such as by promoting a particular observational study (one which Goldacre describes as "little more than a customer-satisfaction survey") as if it were more informative than a series of randomized controlled trials.
Referring specifically to homeopathy, the British House of Commons Science and Technology Committee has stated:
In our view, the systematic reviews and meta-analyses conclusively demonstrate that homeopathic products perform no better than placebos. The Government shares our interpretation of the evidence.
In the Committee's view, homeopathy is a placebo treatment and the Government should have a policy on prescribing placebos. The Government is reluctant to address the appropriateness and ethics of prescribing placebos to patients, which usually relies on some degree of patient deception. Prescribing of placebos is not consistent with an informed patient choice – which the Government claims is very important – as it means patients do not have all the information needed to make choice meaningful.
Beyond ethical issues and the integrity of the doctor-patient relationship, prescribing pure placebos is bad medicine. Their effect is unreliable and unpredictable and cannot form the sole basis of any treatment on the NHS.
The National Center for Complementary and Alternative Medicine of the United States' National Institutes of Health states:

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Homeopathy is a controversial topic in complementary medicine research. A number of the key concepts of homeopathy are not consistent with fundamental concepts of chemistry and physics. For example, it is not possible to explain in scientific terms how a preparation containing little or no active ingredient can have any effect. This, in turn, creates major challenges to the rigorous clinical investigation of homeopathic preparations. For example, one cannot confirm that an extremely dilute preparation contains what is listed on the label, or develop objective measures that show effects of extremely dilute preparations in the human body.
Ben Goldacre noted that in the early days of homeopathy, when medicine was dogmatic and frequently worse than doing nothing, homeopathy at least failed to make matters worse:
During the 19th-century cholera epidemic, death rates at the London Homeopathic Hospital were three times lower than at the Middlesex Hospital. Homeopathic sugar pills won't do anything against cholera, of course, but the reason for homeopathy's success in this epidemic is even more interesting than the placebo effect: at the time, nobody could treat cholera. So, while hideous medical treatments such as blood-letting were actively harmful, the homeopaths' treatments at least did nothing either way.
=== In lieu of standard medical treatment ===
On clinical grounds, patients who choose to use homeopathy in preference to normal medicine risk missing timely diagnosis and effective treatment, thereby worsening the outcomes of serious conditions. Critics of homeopathy have cited individual cases of patients of homeopathy failing to receive proper treatment for diseases that could have been easily diagnosed and managed with conventional medicine and who have died as a result, and the "marketing practice" of criticizing and downplaying the effectiveness of mainstream medicine. Homeopaths claim that use of conventional medicines will "push the disease deeper" and cause more serious conditions, a process referred to as "suppression". Some homeopaths (particularly those who are non-physicians) advise their patients against immunization. Some homeopaths suggest that vaccines be replaced with homeopathic "nosodes", created from biological materials such as pus, diseased tissue, bacilli from sputum or (in the case of "bowel nosodes") faeces. While Hahnemann was opposed to such preparations, modern homeopaths often use them although there is no evidence to indicate they have any beneficial effects. Cases of homeopaths advising against the use of anti-malarial drugs have been identified. This puts visitors to the tropics who take this advice in severe danger, since homeopathic preparations are completely ineffective against the malaria parasite. Also, in one case in 2004, a homeopath instructed one of her patients to stop taking conventional medication for a heart condition, advising her on June 22, 2004, to "Stop ALL medications including homeopathic", advising her on or around August 20 that she no longer needed to take her heart medication, and adding on August 23, "She just cannot take ANY drugs – I have suggested some homeopathic remedies ... I feel confident that if she follows the advice she will regain her health." The patient was admitted to hospital the next day, and died eight days later, the final diagnosis being "acute heart failure due to treatment discontinuation".
In 1978, Anthony Campbell, then a consultant physician at the Royal London Homeopathic Hospital, criticized statements by George Vithoulkas claiming that syphilis, when treated with antibiotics, would develop into secondary and tertiary syphilis with involvement of the central nervous system, saying that "The unfortunate layman might well be misled by Vithoulkas' rhetoric into refusing orthodox treatment".
Vithoulkas' claims echo the idea that treating a disease with external medication used to treat the symptoms would only drive it deeper into the body and conflict with scientific studies, which indicate that penicillin treatment produces a complete cure of syphilis in more than 90% of cases.
A 2006 review by W. Steven Pray of the College of Pharmacy at Southwestern Oklahoma State University recommends that pharmacy colleges include a required course in unproven medications and therapies, that ethical dilemmas inherent in recommending products lacking proven safety and efficacy data be discussed, and that students should be taught where unproven systems such as homeopathy depart from evidence-based medicine.
In an article entitled "Should We Maintain an Open Mind about Homeopathy?" published in the American Journal of Medicine, Michael Baum and Edzard Ernst – writing to other physicians – wrote that "Homeopathy is among the worst examples of faith-based medicine... These axioms [of homeopathy] are not only out of line with scientific facts but also directly opposed to them. If homeopathy is correct, much of physics, chemistry, and pharmacology must be incorrect...".
In 2013, Mark Walport, the UK Government Chief Scientific Adviser and head of the Government Office for Science, had this to say: "My view scientifically is absolutely clear: homoeopathy is nonsense, it is non-science. My advice to ministers is clear: that there is no science in homoeopathy. The most it can have is a placebo effect – it is then a political decision whether they spend money on it or not." His predecessor, John Beddington, referring to his views on homeopathy being "fundamentally ignored" by the Government, said: "The only one [view being ignored] I could think of was homoeopathy, which is mad. It has no underpinning of scientific basis. In fact, all the science points to the fact that it is not at all sensible. The clear evidence is saying this is wrong, but homoeopathy is still used on the NHS."
== References ==
== External links ==
The evidence for homeopathy (by Robert Hahn) at Homeopath UK

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Evidence of absence is evidence of any kind that suggests something is missing or that it does not exist. What counts as evidence of absence has been a subject of debate between scientists and philosophers. It is often distinguished from absence of evidence.
== Overview ==
Evidence of absence and absence of evidence are similar but distinct concepts. This distinction is captured in the aphorism "Absence of evidence is not evidence of absence." This antimetabole is often attributed to Martin Rees or Carl Sagan, but a version appeared as early as 1888 in a writing by William Wright. In Sagan's words, the expression is a critique of the "impatience with ambiguity" exhibited by appeals to ignorance. Despite what the expression may seem to imply, a lack of evidence can be informative. For example, when testing a new drug, if no harmful effects are observed then this suggests that the drug is safe. This is because, if the drug were harmful, evidence of that fact can be expected to turn up during testing. The expectation of evidence makes its absence significant.
As the previous example shows, the difference between evidence that something is absent (e.g., an observation that suggests there were no dragons here today) and simple absence of evidence (e.g., no careful research has been done) can be nuanced. Indeed, scientists will often debate whether an experiment's result should be considered evidence of absence, or if it remains absence of evidence. The debate regards whether the experiment would have detected the phenomenon of interest if it were there.
The argument from ignorance for "absence of evidence" is not necessarily fallacious, for example, that a potentially life-saving new drug poses no long-term health risk unless proved otherwise. On the other hand, were such an argument to rely imprudently on the lack of research to promote its conclusion, it would be considered an informal fallacy whereas the former can be a persuasive way to shift the burden of proof in an argument or debate.
== Science ==
In carefully designed scientific experiments, null results can be interpreted as evidence of absence. Whether the scientific community will accept a null result as evidence of absence depends on many factors, including the detection power of the applied methods, the confidence of the inference, as well as confirmation bias within the community. For instance in amnesia studies, the absence of behavior indicative of memory is sometimes interpreted as the absence of the memory trace; however, certain researchers consider this interpretation flawed as the memory impairment may be temporary due to deficits in recall. Alternatively, the memory trace be latent and demonstrable via its indirect effects on new learning. Michael Davis, researcher at Emory University, argues that complete erasure can only be confidently inferred if all of the biological events that occurred when the memory was formed revert to their original status. Davis contends that because making these measurements in a complex organism is implausible, the concept of complete memory erasure (what he deems "strong form of forgetting") is not useful scientifically.
== Law ==
In many legal systems, a lack of evidence for a defendant's guilt is sufficient for acquittal. This is because of the presumption of innocence and the belief that it is worse to convict an innocent person than to let a guilty one go free.
On the other hand, the absence of evidence in the defendant's favor (e.g. an alibi) can make their guilt seem more likely. A jury can be persuaded to convict because of "evidentiary lacunae", or a lack of evidence they expect to hear.
== Proving a negative ==
A negative claim is a colloquialism for an affirmative claim that asserts the non-existence or exclusion of something. Proofs of negative claims are common in mathematics. Such claims include Euclid's theorem that there is no largest prime number, and Arrow's impossibility theorem. There can be multiple claims within a debate, nevertheless, whoever makes a claim usually carries the burden of proof regardless of positive or negative content in the claim.
A negative claim may or may not exist as a counterpoint to a previous claim. A proof of impossibility or an evidence of absence argument are typical methods to fulfill the burden of proof for a negative claim.
Philosopher Steven Hales argues that typically one can logically be as confident with the negation of an affirmation. Hales says that if one's standards of certainty leads them to say "there is never 'proof' of non-existence", then they must also say that "there is never 'proof' of existence either". Hales argues that there are many cases where we may be able to prove something does not exist with as much certainty as proving something does exist. A similar position is taken by philosopher Stephen Law who highlights that rather than focusing on the existence of "proof", a better question would be whether there is any reasonable doubt for existence or non-existence.
== See also ==
Argument from ignorance
Argument from silence
Contraposition
Probatio diabolica
Proof by exhaustion
== References ==

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Evidentialism is a thesis in epistemology which states that one is justified to believe something if and only if that person has evidence which supports said belief. Evidentialism is, therefore, a thesis about which beliefs are justified and which are not.
For philosophers Richard Feldman and Earl Conee, evidentialism is the strongest argument for justification because it identifies the primary notion of epistemic justification. They argue that if a person's attitude towards a proposition fits their evidence, then their doxastic attitude for that proposition is epistemically justified. Feldman and Conee offer the following argument for evidentialism as an epistemic justification:
(EJ) Doxastic attitude D toward proposition p is epistemically justified for S at t if and only if having D toward p fits the evidence.
For Feldman and Conee one's doxastic attitude is justified if it fits one's evidence. EJ is meant to show the idea that justification is characteristically epistemic. This idea makes justification dependent on evidence.
Feldman and Conee believe that because objections to EJ have become so prominent their defense for it is appropriate. The theses that object EJ are implying that epistemic justification is dependent upon the "cognitive capacities of an individual or upon the cognitive processes or information-gatherings practices that lead to an attitude." For Feldman and Conee, EJ is in contrast to these theses; EJ contends that the epistemic justification for an attitude is only dependent upon evidence.
== Criticism ==
Critics of evidentialism sometimes reject the claim that a conclusion is justified only if one's evidence supports that conclusion. A typical counterexample goes like this. Suppose, for example, that Babe Ruth approaches the batter's box believing that he will hit a home run despite his current drunkenness and overall decline in performance in recent games. He realizes that, however unlikely it is that his luck will change, it would increase his chances of hitting a home run if he maintains a confident attitude. In these circumstances, critics of evidentialism argue that his belief that p = Babe Ruth will hit a home run is justified, even though his evidence does not support this belief.
Evidentialists may respond to this criticism by forming a distinction between pragmatic or prudential justification and epistemic justification. In Babe Ruth's case, it is pragmatically justified that he believe p, but it is nevertheless epistemically unjustified: though the belief may be justified for the purpose of promoting some other goal (a successful at bat, in Ruth's case), it is not justified relative to the purely epistemic goal of having beliefs that are most likely to be true.
A similar response follows the criticism that evidentialism implies all faith-based beliefs are unjustified. For example, fideism claims that evidence is irrelevant to religious beliefs and that attempts to justify religious beliefs in such a way are misguided. Superficially, fideism and evidentialism have mutually exclusive takes on religious beliefs, but evidentialists use the term "justification" in a much weaker sense than the one in which fideists most likely use it. Evidentialism merely defines the epistemic condition of a belief.
Although evidentialism states that the content of the evidence does not matter, only that it constitutes valid justification towards some proposition, a skeptical criticism may be levelled at evidentialism from uncertainty theories. One's evidence may be objectively disproved at some point or it may be the case that one can never have absolute certainty of one's evidence. Given the logic of arguments concerning principles of uncertainty and randomness, skepticism towards knowledge merely becomes skepticism towards valid justification.
Likewise, some say that the human mind is not naturally inclined to form beliefs based on evidence, viz. cognitive dissonance. While this may be the case, evidentialists admit, evidentialism is only meant to separate justified beliefs from unjustified beliefs. One can believe that evidentialism is true yet still maintain that the human mind is not naturally inclined to form beliefs based on evidence. He would simply have to conclude that the mind is not naturally inclined to form justified beliefs.
== Infinite regress argument ==
Evidentialism also faces a challenge from the infinite regress argument. This argument begins with the observation that, normally, one's supporting evidence for a belief consists of other beliefs. However, it seems that these other beliefs can do the job of justifying only if they themselves are already justified. And evidentialism demands that these supporting beliefs be justified by still further evidence if they are to be justified themselves. But this same reasoning would apply to the new, deeper level of supporting beliefs: they can only justify if they're themselves justified, and evidentialism therefore demands an even deeper level of supporting belief. According to this argument, a justified belief requires an endless supply of reasons. Some philosophers such as Thomas Nagel posit that this is an absurd conclusion.
In general, responses to this argument can be classified in the following ways:

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Foundationalism: There exist beliefs that are justified, but not because they are based on any other beliefs. These are called properly basic beliefs, and they are the foundation upon which all other justified beliefs ultimately rest.
Coherentism: Justified beliefs are all evidentially supported by other beliefs, but an infinite set of beliefs is not generated, because the chains of evidential support among beliefs is allowed to move in a circle. On the resulting picture, a person's belief is justified when it fits together with the person's other beliefs in a coherent way in which the person's various beliefs mutually support one another.
A modest reasoner subset of Coherentism would insist that all justifiable beliefs be statements about "some objects" since the negation/complement of a some statement is another some statement.
Skepticism: There cannot be any justified beliefs.
A modest reasoner subset of Scepticism like the subset of Coherentism would likewise insist and define all justifiable beliefs be statements about "some objects" since the negation/complement of a some statement is another some statement.
Infinitism: Aside from these responses, some philosophers have said that evidential chains terminate in beliefs that are not justified. Others have said that, indeed, there can exist infinite chains of reasons.
Of the main responses, coherentism and skepticism are clearly consistent with evidentialism. Coherentism allows evidential support for all of our justified beliefs in the face of the regress argument by allowing for circular chains of evidential support among beliefs. And the skeptic here is utilizing an evidentialist demand to arrive at her skeptical conclusion.
But because the resulting skepticism is so sweeping and devastating, and because so many reject the legitimacy of the circular reasoning embraced by the coherentist, foundationalism is the favored response of many philosophers to the regress argument. And foundationalism does not so clearly fit together with evidentialism. At first glance, at least, the "basic" beliefs of the foundationalist would appear to be counterexamples to the evidentialist's thesis, in that they are justified beliefs that are not rational because they are not supported by deeper evidence.
== Non-evidentialist theories of knowledge and justification ==
Many contemporary epistemologists reject the view that evidential support is the whole story about the justification of beliefs. While no sensible epistemologists generally urge people to disregard their evidence when forming beliefs, many believe that a more complete theory would introduce considerations about the processes that initiate and sustain beliefs. An example of one such theory is reliabilism. The most influential proponent of reliabilism is Alvin Goldman. According to a crude form of reliabilism, S is justified in believing p if and only if S's belief in p is caused by a reliable process—a process that generally leads to true beliefs. Some of these reliable processes may require the processing of evidence; many others won't. So, Goldman would argue, evidentialism, on which the justification of a belief always turns completely on the issue of the belief's evidential support, is false. Likewise, evidentialism will be rejected by more sophisticated versions of reliabilism, some of which will allow evidence an important but limited role, as opposed to the all-encompassing role assigned to it by evidentialism.
Other non-evidentialist theories include: the causal theory, according to which S knows p if and only if S's belief in p is causally connected in an appropriate way with S's believing p; and Robert Nozick's truth tracking theory, according to which S knows p if and only if (i) p is true, (ii) S believes p, (iii) S's attitude toward p tracks the truth value of p in that, when p is not true, S does not believe p and when p is true, S does believe p.
Another alternative perspective, promoted by David Hume's 18th-century opponent, Presbyterian philosopher Thomas Reid, and perhaps hinted at by Hume himself, at least in some moods (though this is a very controversial issue in interpreting Hume), has it that some of our "natural" beliefs—beliefs we are led to form by natural features of the human constitution—have what can be called an "innocent-until-proven-guilty" status. Contrary to evidentialism, they can be justified in the absence of any effective evidence that supports them. They are justified just so long as one doesn't have good reason to think them false.
A new account of the extent of our evidence is Timothy Williamson's claim that E=K: one's evidence is what one knows. Going by the "letter of the law," Williamson's resulting theory is not contrary to, but is rather an instance of, evidentialism. By allowing our evidence to encompass everything we know, Williamson is able to give thoroughly evidentialist accounts of many important epistemological concepts. But, traditionally, evidentialists have presupposed much more restrictive accounts of what our evidence is. Thus, Williamson's theory is opposed to the spirit of much traditional evidentialism, primarily because it turns evidentialism from an internalist account of justification to an externalist account (due to the factive nature of knowledge.) However, Williamson's work may point to a quite general way to modify traditional evidentialism to make it better able to meet the challenges it faces: whether or not one goes so far as to accept that E=K, broadening one's view of what constitutes our evidence may provide a way to address many of the objections to evidentialism, especially to those disinclined to swallow skeptical consequences of a view
== Notes ==
== References ==
Conee; Feldman (2004), Evidentialism, Oxford University Press.
== External links ==
Fieser, James; Dowden, Bradley (eds.). "Evidentialism". Internet Encyclopedia of Philosophy. ISSN 2161-0002. OCLC 37741658. by Dan Mittag of the University of Rochester
Kelly, Thomas. "Evidence". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy. ISSN 1095-5054. OCLC 429049174.
Evidentialism at the Indiana Philosophy Ontology Project
Evidentialism at PhilPapers

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In linguistics, evidentiality is, broadly, the indication of the nature of evidence for a given statement; that is, whether evidence exists for the statement and if so, what kind. An evidential (also verificational or validational) is the particular grammatical element (affix, clitic, or particle) that indicates evidentiality. Languages with only a single evidential have had terms such as mediative, médiatif, médiaphorique, and indirective used instead of evidential.
Evidentiality may be direct or indirect: direct evidentials are used to describe information directly perceived by the speaker through vision as well as other sensory experiences while indirect evidentials consist of the other grammatical markers for evidence such as quotatives and inferentials.
== Introduction ==
All languages have some means of specifying the source of information. European languages (such as Germanic and Romance languages) often use modal verbs (Spanish: deber de, Dutch: zouden, Danish: skulle, German: sollen) or other lexical words (adverbials, English: reportedly) or phrases (English: it seems to me).
Some languages have a distinct grammatical category of evidentiality that is required to be expressed at all times. In contrast, the elements in European languages indicating the information source are optional and usually do not indicate evidentiality as their primary function; thus, they do not form a grammatical category. The obligatory elements of grammatical evidentiality systems may be translated into English, variously, as I hear that, I see that, I think that, as I hear, as I can see, as far as I understand, they say, it is said, it seems, it seems to me that, it looks like, it appears that, it turns out that, alleged, stated, allegedly, reportedly, obviously, etc.
Alexandra Aikhenvald (2004) reports that about a quarter of the world's languages have some type of grammatical evidentiality. Laura Mazzoni has since conducted a preliminary study on evidentiality in Italian Sign Language (LIS).
Grammatical evidentiality may be expressed in different forms depending on the language, such as through affixes, clitics, or particles. For example, Japanese has inferential evidentials and reportive markers that are realized as suffixes on a variety of mainly verbal predicates, and as grammaticalized nouns. As another example, Eastern Pomo uses four evidential suffixes that are added to verbs: -ink’e (nonvisual sensory), -ine (inferential), -·le (hearsay), and -ya (direct knowledge).
Many languages with grammatical evidentiality mark evidentiality independently from tense-aspect or epistemic modality, which is the speaker's evaluation of the information, i.e. whether it is reliable, uncertain, probable.
The use of evidentiality has pragmatic implications. In languages that do not mark evidentiality distinctly from epistemic modality, for example, a person who makes a false statement qualified as a belief may be considered mistaken, while a person who makes a false statement qualified as a personally observed fact will probably be considered to have lied. More generally, a speaker of a language that does have obligatory grammatical evidentiality is required to cognitively engage with the source of their belief of any statement in a manner that the speaker of languages without obligatory evidentiality may gloss over.
In some languages, evidential markers also serve other purposes, such as indicating the speaker's attitude towards, or belief in, the statement. Usually a direct evidential marker may serve to indicate that the speaker is certain about the event stated. Using an indirect evidential marker, such as one for hearsay or reported information, may indicate that the speaker is uncertain about the statement, or doesn't want to take responsibility for its truth. A "hearsay" evidential may then have the undertone of "that's what they say; whether or not it's true is nothing I can take responsibility for". In other languages, this is not the case. Therefore, one should distinguish between such evidential markers that only mark source of knowledge, and such evidential markers that serve other functions, such as marking epistemic modality.
Evidentials can also be used to "deflect culpability" in a statement. In his dissertation on Nanti, a Peruvian Amazonian language, Lev Michael refers to an example in which a young girl is accidentally burned, and a community member questions her mother about how it happened. Her mother uses the evidential marker ka which translates to "presumably," to deflect responsibility for the girl's mistake.
Some languages are borderline cases. For example, the Romance languages are mostly like English in not having grammatical evidentiality, but do have a conditional mood which has three uses: conditions, future-in-the-past, and hearsay. Thus in journalistic French, there is frequently a distinction between Il a reconnu sa culpabilité and Il aurait reconnu sa culpabilité: both translate to "He has admitted his guilt," but with an implication of certainty with the first, and the idea of "reportedly" with the second. The same happens in Spanish (Él ha reconocido su culpa vs. Él habría reconocido su culpa) and in Portuguese (Ele reconheceu sua culpa vs. Ele teria reconhecido sua culpa).
Aikhenvald identified five semantic categories that recurrently occur across languages of the world:
Visual Sensory
Non-Visual Sensory
Inferentials
Hearsay Reportatives
Quotative Reportatives
No language has been reported to have special forms for smell, taste or feeling although these may be covered by non-visual evidentials.
== Types according to Aikhenvald ==
Following the typology of Alexandra Aikhenvald, there are two broad types of evidential marking:
indirectivity marking ("type I")
evidential marking ("type II")
The first type (indirectivity) indicates whether evidence exists for a given statement, but does not specify what kind of evidence. The second type (evidentiality proper) specifies the kind of evidence (such as whether the evidence is visual, reported, or inferred).

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=== Indirectivity (type I) ===
Indirectivity (also known as inferentiality) systems are common in Uralic and Turkic languages. These languages indicate whether evidence exists for a given source of information; thus, they contrast direct information (reported directly) and indirect information (reported indirectly, focusing on its reception by the speaker/recipient). Unlike the other evidential "type II" systems, an indirectivity marking does not indicate information about the source of knowledge: it is irrelevant whether the information results from hearsay, inference, or perception; however, some Turkic languages distinguish between reported indirect and non-reported indirect, see Johanson 2003, 2000 for further elaboration. This can be seen in the following Turkish verbs:
In the word geldi, the unmarked suffix -di indicates past tense. In the second word gelmiş, the suffix -miş also indicates past tense but indirectly. It may be translated into English with the added phrases 'obviously', 'apparently' or 'as far as I understand'. The direct past tense marker -di is unmarked (or neutral) in the sense that whether or not evidence exists supporting the statement is not specified.
=== Evidentiality (type II) ===
The other broad type of evidentiality systems ("type II") specifies the nature of the evidence supporting a statement. These kinds of evidence can be divided into such categories as:
Sensory
Visual
Non-visual
Inferential
Assumed
Reportative
Hearsay
Quotative
Sensory evidentials can often be divided into different types. Some languages mark visual evidence differently from nonvisual evidence that is heard, smelled, or felt. The Kashaya language has a separate auditory evidential.
An inferential evidential indicates information was not personally experienced but was inferred from indirect evidence. Some languages have different types of inferential evidentials. Some of the inferentials found indicate:
Information inferred by direct physical evidence
Information inferred by general knowledge
Information inferred/assumed because of speaker's experience with similar situations
Past deferred realization
In many cases, different inferential evidentials also indicate epistemic modality, such as uncertainty or probability (see epistemic modality below). For example, one evidential may indicate that the information is inferred but of uncertain validity, while another indicates that the information is inferred but unlikely to be true.
Reportative evidentials indicate that the information was reported to the speaker by another person. A few languages distinguish between hearsay evidentials and quotative evidentials. Hearsay indicates reported information that may or may not be accurate. A quotative indicates the information is accurate and not open to interpretation, i.e., is a direct quotation. An example of a reportative from Shipibo (-ronki):
==== Typology of evidentiality systems ====
The following is a brief survey of evidential systems found in the languages of the world as identified in Aikhenvald (2004). Some languages only have two evidential markers while others may have six or more. The system types are organized by the number of evidentials found in the language. For example, a two-term system (A) will have two different evidential markers; a three-term system (B) will have three different evidentials. The systems are further divided by the type of evidentiality that is indicated (e.g. A1, A2, A3, etc.). Languages that exemplify each type are listed in parentheses.
The most common system found is the A3 type.
Two-term systems:
A1. witness, nonwitness (e.g. Jarawara, Yukaghir languages, Mỹky, Godoberi, Kalasha-mun, Khowar, Yanam)
A2. nonfirsthand, everything else (e.g. Abkhaz, Mansi, Khanty, Nenets, Enets, Selkup, Northeast Caucasian languages)
A3. reported, everything else (e.g. Turkic languages, Tamil, Enga, Tauya, Lezgian, Kham, Estonian, Livonian, Tibeto-Burman languages, several South American languages)
Three-term systems:
B1. visual sensory, inferential, reportative (e.g. Aymara, Shastan languages, Qiang languages, Maidu, most Quechuan languages, Northern Embera languages)
B2. visual sensory, nonvisual sensory, inferential (e.g. Washo)
B3. nonvisual sensory, inferential, reportative (e.g. Retuarã, Northern Pomo)
B4. witness (direct), nonwitness (indirect), inferential, reportative (e.g. Tsezic and Dagestanian languages)
Four-term systems:
C1. visual sensory, nonvisual sensory, inferential, reportative (e.g. Tariana, Xamatauteri, Eastern Pomo, East Tucanoan languages)
C2. visual sensory, inferential #1, inferential #2, reportative (e.g. Tsafiki, Pawnee, Ancash Quechua)
C3. nonvisual sensory, inferential #1, inferential #2, reportative (e.g. Wintu)
C4. visual sensory, inferential, reportative #1, reportative #2 (e.g. Southeastern Tepehuan)
C5. witness (non-subjective, non-renarrative), inferential (subjective, non-renarrative), renarrative (non-subjective, renarrative), dubitative (subjective, renarrative) (e.g. Bulgarian)
Five-plus term systems:
visual sensory, nonvisual sensory, inferential, reportative, assumed (e.g. Tuyuca, Tucano)
witness, inferential, reportative, assumed, "internal support" (e.g. Nambikwaran languages)
visual sensory, nonvisual sensory, inferential, reported, heard from known source, direct participation (e.g. Fasu)
nonvisual sensory, inferential #1, inferential #2, inferential #3, reportative (e.g. Western Apache)
inferential, anticipation, performative, deduction, induction, hearsay, direct observation, opinion, assumed, "to know by culture", "to know by internal" (Lojban)
== Evidentiality marking and other categories ==
Evidential systems in many languages are often marked simultaneously with other linguistic categories. For example, according to Aikhenvald, a given language may use the same element to mark both evidentiality and mirativity, i.e., unexpected information. She claims that this is the case of Western Apache where the post-verbal particle lą̄ą̄ primarily functions as a mirative but also has a secondary function as an inferential evidential. This phenomenon of evidentials developing secondary functions, or other grammatical elements such as miratives and modal verbs developing evidential functions is fairly widespread. The following types of mixed systems have been reported:
evidentiality with mirativity
evidentiality with tense-aspect
evidentiality with modality (this is discussed in the next section below)
In addition to the interactions with tense, modality, and mirativity, the usage of evidentials in some languages may also depend on the clause type, discourse structure, and/or linguistic genre.
However, despite the intersection of evidentiality systems with other semantic or pragmatic systems (through grammatical categories), Aikhenvald believes that several languages do mark evidentiality without any grammatical connection to these other semantic/pragmatic systems. More explicitly stated, she believes that there are modal systems which do not express evidentiality, and evidential systems which do not express modality. Likewise, there are mirative systems which do not express evidentiality, and evidential systems which do not express mirativity.
Aside from those, egophoricity may interact with evidentiality as well.

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=== Tense ===
Some languages may only distinguish between direct and indirect evidentials in the past tense. This is the case for Georgian (Kartvelian), Turkish (Turkic), Komi-Zyrian (Finno-Ugric), Haida (a language isolate in British Columbia and Alaska), and Ika (Chibchan).
=== Epistemic modality ===
Evidentiality is often considered to be a sub-type of epistemic modality (see, for example, Palmer 1986, Kiefer 1994). Other linguists consider evidentiality (marking the source of information in a statement) to be distinct from epistemic modality (marking the degree of confidence in a statement). An English example:
I see that he is coming. (evidential)
I know that he is coming. (epistemic)
For instance, de Haan states that evidentiality asserts evidence while epistemic modality evaluates evidence and that evidentiality is more akin to a deictic category marking the relationship between speakers and events/actions (like the way demonstratives mark the relationship between speakers and objects; see also Joseph 2003). Aikhenvald (2003) finds that evidentials may indicate a speaker's attitude about the validity of a statement but this is not a required feature of evidentials. Additionally, she finds that evidential-marking may co-occur with epistemic-marking, but it may also co-occur with aspectual/tense or mirative marking.
Considering evidentiality as a type of epistemic modality may only be the result of analyzing non-European languages in terms of the systems of modality found in European languages. For example, the modal verbs in Germanic languages are used to indicate both evidentiality and epistemic modality (and are thus ambiguous when taken out of context). Other (non-European) languages clearly mark these differently. De Haan (2001) finds that the use of modal verbs to indicate evidentiality is comparatively rare (based on a sample of 200 languages).
=== Clause type ===
Evidential categories are more likely to be marked in a main declarative clause than in the other types of clauses. In some languages, however, evidential forms may appear in questions or commands as well.
=== Terminology ===
Although some linguists have proposed that evidentiality should be considered separately from epistemic modality, other linguists conflate the two. Because of this conflation, some researchers use the term evidentiality to refer both to the marking of the knowledge source and the commitment to the truth of the knowledge.
== In English (not grammaticalized) ==
Evidentiality is not considered a grammatical category in English because it is expressed in diverse ways and is always optional. In contrast, many other languages (including Quechua, Aymara, and Yukaghir) require the speaker to mark the main verb or the sentence as a whole for evidentiality, or offer an optional set of affixes for indirect evidentiality, with direct experience being the default assumed mode of evidentiality.
Consider these English sentences:
I am hungry.
Bob is hungry.
We are unlikely to say the second unless someone (perhaps Bob himself) has told us that Bob is hungry (We might still say it for someone incapable of speaking for themself, such as a baby or a pet). If we are simply assuming that Bob is hungry based on the way he looks or acts, we are more likely to say something like:
Bob looks hungry.
Bob seems hungry.
Bob would be hungry by now.
Bob must be hungry by now.
Here, the fact that we are relying on sensory evidence, rather than direct experience, is conveyed by our use of the word look or seem.
Another situation in which the evidential modality is expressed in English is in certain kinds of predictions, namely those based on the evidence at hand. These can be referred to as "predictions with evidence". Examples:
Look at those clouds! It's going to rain! (Compare "It will rain!").
=== Possible exceptions ===
The suffix "-ish" can be considered to be a grammaticalized marker of uncertainty.
== Western history of the concept ==
The notion of evidentiality as obligatory grammatical information was first made apparent in 1911 by Franz Boas in his introduction to The Handbook of American Indian Languages in a discussion of Kwakiutl and in his grammatical sketch of Tsimshianic. The term evidential was first used in the current linguistic sense by Roman Jakobson in 1957 in reference to Balkan Slavic (Jacobsen 1986:4; Jakobson 1990) with the following definition:
"EnEns/Es evidential is a tentative label for the verbal category which takes into account three events — a narrated event (En), a speech event (Es), and a narrated speech event (Ens). The speaker reports an event on the basis of someone else's report (quotative, i.e. hearsay evidence), of a dream (revelative evidence), of a guess (presumptive evidence) or of his own previous experience (memory evidence)."
Jakobson also was the first to clearly separate evidentiality from grammatical mood. By the middle of the 1960s, evidential and evidentiality were established terms in linguistic literature.
Systems of evidentiality have received focused linguistic attention only relatively recently. The first major work to examine evidentiality cross-linguistically is Chafe & Nichols (1986). A more recent typological comparison is Aikhenvald (2004).
== See also ==
Epistemology – Philosophical study of knowledge
Linguistic modality – Phenomenon whereby language is used to discuss possible situationsPages displaying short descriptions of redirect targets
Epistemic modality – Type of linguistic modality
Mirativity – Grammatical category which conveys surprise
Egophoricity – Linguistic encoding of personal knowledge
Grammatical mood – Grammatical feature of verbs
Evidence theory – Mathematical framework to model epistemic uncertaintyPages displaying short descriptions of redirect targets
== References ==

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== Further reading ==
Aikhenvald, Alexandra Y.; & Dixon, R. M. W. (1998). Evidentials and areal typology: A case-study from Amazonia. Language Sciences, 20, 241–257.
Aikhenvald, Alexandra Y.; & Dixon, R. M. W. (Eds.). (2003). Studies in evidentiality. Typological studies in language (Vol. 54). Amsterdam: John Benjamins Publishing Company. ISBN 90-272-2962-7; ISBN 1-58811-344-2.
Aikhenvald, Alexandra Y.; & Dixon, R. M. W. (Eds.). (2014) The Grammar of Knowledge: A Cross-Linguistic Typology. Oxford University Press. ISBN 978-0-19-870131-6
Blakemore, D. (1994). Evidence and modality. In R. E. Asher (Ed.), The Encyclopedia of language and linguistics (pp. 1183–1186). Oxford: Pergamon Press. ISBN 0-08-035943-4.
Chafe, Wallace L.; & Nichols, Johanna. (Eds.). (1986). Evidentiality: The linguistic encoding of epistemology. Norwood, NJ: Ablex.
Comrie, Bernard. (2000). Evidentials: Semantics and history. In L. Johanson & B. Utas (Eds.).
de Haan, Ferdinand (2013b), "Coding of Evidentiality", in Dryer, Matthew S.; Haspelmath, Martin (eds.), WALS Online (v2020.3), retrieved February 3, 2024
Faust, Norma. (1973). Lecciones para el aprendizaje del idioma shipibo-conibo [Lessons for learning the Shipibo-Conibo language]. Lima: Summer Institute of Linguistics.
Guentchéva, Zlatka. (1996a). Introduction. In Z. Guentchéva (Ed.) (pp. 11–18).
Guentchéva, Zlatka (Ed.). (1996b). L’Énonciation médiatisée. Bibliothèque de l’information grammaticale. Louvain: Éditions Peeters. ISBN 90-6831-861-6; ISBN 2-87723-244-1.
Johanson, Lars. (2000). Turkic indirectives. In L. Johanson & B. Utas (Eds.) (pp. 61–87).
Jacobsen, W. H. Jr. (1986). The heterogeneity of evidentials in Makah. In W. L. Chafe & J. Nichols (Eds.) (pp. 3–28).
Jakobson, Roman. (1990). Shifters and verbal categories. In On language (pp. 386–392). Cambridge, MA: Harvard University Press. (Original work published 1957).
Johanson, Lars. (2003). Evidentiality in Turkic. In A. Y. Aikhenvald & R. M. W. Dixon (Eds.) (pp. 273–290).
Johanson, Lars; & Utas, Bo (Eds.). (2000). Evidentials: Turkic, Iranian and neighboring languages. Berlin: Mouton de Gruyter. ISBN 3-11-016158-3.
Joseph, Brian D. (2003). Evidentials: Summation, questions, prospects. In A. Y. Aikhenvald & R. M. W. Dixon (Eds.) (pp. 307–327).
Kiefer, Ferenc. (1994). Modality. In R. E. Asher (Ed.), The Encyclopedia of language and linguistics (pp. 2515–2520). Oxford: Pergamon Press.
LaPolla, Randy J. (2003). Evidentiality in Qiang. In A. Y. Aikhenvald & R. M. W. Dixon (Eds.) (pp. 63–78).
Maslova, Elena. (2003). Evidentiality in Yukaghir. In A. Y. Aikhenvald & R. M. W. Dixon (Eds.) (pp. 237–241).
Noël, Dirk. (2001). The passive matrices of English infinitival complement clauses: Evidentials on the road to auxiliarihood? Studies in Language, 25, 255–296.
Palmer, F. R. (1986). Mood and modality. Cambridge: Cambridge University Press. ISBN 0-521-26516-9, ISBN 0-521-31930-7. (2nd ed. published 2001).
Palmer, F. R. (1994). Mood and modality. In R. E. Asher (Ed.), The Encyclopedia of language and linguistics (pp. 2535–2540). Oxford: Pergamon Press.
Slobin, Dan Isaac; Aksu, Ayhan A. (1982). "Tense, Aspect and Modality in the Use of the Turkish Evidential" (PDF). In Hopper, Paul J. (ed.). Tense-Aspect: Between semantics & pragmatics. Typological Studies in Language. Vol. 1. John Benjamins. p. 185. doi:10.1075/tsl.1.13slo. ISBN 978-90-272-2865-9. Archived from the original on April 2, 2024.
Speas, Peggy. (2010) 'Evidentials as Generalized Functional Heads.' in A.M. diScuillo, ed. Interface Legibility at the Edge. Oxford University Press.
Willet, Thomas L. (1988). A cross-linguistic survey of the grammaticalization of evidentiality. Studies in Language, 12, 51–97.
== External links ==
Language & Power (Evidentiality)
Ferdinand de Haan's research on evidentiality
Evidentiality bibliography
world map of the language distribution of evidentiality
Semantics: Modality and Evidentiality
Evidentiality in Dena’ina Athabascan
review of Aikhenvald & Dixon (2003) Deprecated link archived 2013-01-12 at archive.today (Linguist List)
review of Aikhenvald (2004) Deprecated link archived 2013-01-13 at archive.today (Linguist List)

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Exculpatory evidence is evidence favorable to the defendant in a criminal trial that exonerates or tends to exonerate the defendant of guilt. It is the opposite of inculpatory evidence, which tends to present guilt. In many countries, including the United States, police and prosecutors are required to disclose to the defendant exculpatory evidence they possess before the defendant enters a plea (guilty or not guilty). In some countries, such as Germany, the prosecutor has to actively search for both exculpatory and inculpatory circumstances and evidence before filing of action.
Per the Brady v. Maryland (1963) decision, prosecutors in the United States have a duty to disclose exculpatory evidence even if not requested to do so. While the prosecution is not required to search for exculpatory evidence and must disclose only the evidence in its possession, custody, or control, the prosecution's duty is to disclose all information known to any member of its team, e.g., police, investigators, crime labs, et cetera. In Brady v. Maryland, the U.S. Supreme Court held that such a requirement follows from constitutional due process and is consistent with the prosecutor's duty to seek justice. The Brady doctrine is a pretrial discovery rule that was established by the United States Supreme Court in Brady v. Maryland. The rule requires that the prosecution must turn over all exculpatory evidence to the defendant in a criminal case. Exculpatory evidence is evidence that might exonerate the defendant.
== Illustration ==
A victim is murdered by stabbing and a suspect is arrested for the murder. Evidence includes a knife covered with blood found near the victim and the accused found covered in blood at the murder scene. During the investigation, the police interview a witness claiming to have seen the stabbing. The witness makes a statement to the police that another unidentified person committed the crime, not the accused. The witness's statement is exculpatory evidence as it introduces reasonable doubt as to the guilt of the accused. The police either do not believe the witness's account or else find the witness unreliable and choose not to follow up on the lead. The prosecutor is obliged to inform the accused and their attorney of the witness's statement even though the police doubt the witness's version of events. Failure to do so would provide grounds for a motion to dismiss the charges or an appeal of a subsequent guilty verdict.
== See also ==
Brady disclosure
Giglio v. United States (1972)
R v Stinchcombe (1991)
United States v. Williams (1992)
== References ==

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Experimental software engineering involves running experiments on the processes and procedures involved in the creation of software systems, with the intent that the data be used as the basis of theories about the processes involved in software engineering (theory backed by data is a fundamental tenet of the scientific method). A number of research groups primarily use empirical and experimental techniques.
The term empirical software engineering emphasizes the use of empirical studies of all kinds to accumulate knowledge. Methods used include experiments, case studies, surveys, and using whatever data is available.
== Empirical software engineering research ==
In a keynote at the International Symposium on Empirical Software Engineering and Measurement Prof. Wohlin recommended ten commitments that the research community should follow to increase the relevance and impact of empirical software engineering research. However, at the same conference Dr. Ali effectively argued that solely following these will not be enough and we need to do more than just show the evidence substantiating the claimed benefits of our interventions but instead what is required for practical relevance and potential impact is the evidence for cost-effectiveness.
The International Software Engineering Research Network (ISERN) is a global community of research groups who are active in experimental software engineering. Its purpose is to advance the practice of and foster university and industry collaborations within experimental software engineering. ISERN holds annual meetings in conjunction with the International Symposium on Empirical Software Engineering and Measurement (ESEM) conference.
== References ==
== Bibliography ==
Victor Basili, Richard W. Selby, David H. Hutchens, "Experimentation in Software Engineering", IEEE Transactions on Software Engineering, Vol. SE-12, No.7, July 1986
Basili, V.; Rombach, D.; Schneider, K.; Kitchenham, B.; Pfahl, D.; Selby, R. (Eds.),Empirical Software Engineering Issues. Critical Assessment and Future Directions, Springer-Verlag, 2007, ISBN 978-3-540-71300-5.
Barry Boehm, Hans Dieter Rombach, and Marvin V. Zelkowitz (eds.), Foundations of Empirical Software Engineering — The Legacy of Victor R. Basili, Springer-Verlag, 2005, ISBN 3-540-24547-2.
Jones, D. Evidence-based Software Engineering based on the publicly available data, 2020, ISBN 978-1-8382913-0-3
H. Dieter Rombach, Victor R. Basili and Richard W. Selby (eds.), [Experimental Software Engineering Issues: Critical Assessment and Future Directions], Springer-Verlag, 1993, ISBN 3-540-57092-6.

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"Extraordinary claims require extraordinary evidence" (sometimes shortened to ECREE), also known as the Sagan standard, is an aphorism popularized by science communicator Carl Sagan. He used the phrase in his 1979 book Broca's Brain and the 1980 television program Cosmos. It has been described as fundamental to the scientific method and is regarded as encapsulating the basic principles of scientific skepticism.
The concept is similar to Occam's razor in that both heuristics prefer simpler explanations of a phenomenon to more complicated ones. In application, there is some ambiguity regarding when evidence is deemed sufficiently "extraordinary". It is often invoked to challenge data and scientific findings, or to criticize pseudoscientific claims. Some critics have argued that the standard can suppress innovation and affirm confirmation biases.
Philosopher David Hume characterized the principle in his 1748 essay "Of Miracles". Similar statements were made by figures such as Thomas Jefferson in 1808, Pierre-Simon Laplace in 1814, and Théodore Flournoy in 1899. The formulation "extraordinary claims require extraordinary proof" was used a year prior to Sagan, by scientific skeptic Marcello Truzzi.
== Application ==
The aphorism "extraordinary claims require extraordinary evidence", according to psychologist Patrizio Tressoldi, "is at the heart of the scientific method, and a model for critical thinking, rational thought and skepticism everywhere". It has also been described as a "fundamental principle of scientific skepticism". The phrase is often used in the context of paranormal and other pseudoscientific claims. It is also frequently invoked in scientific literature to challenge research proposals, like a new species of Amazonian tapir, biparental inheritance of mitochondrial DNA, or a Holocene "mega-tsunami".
The concept is related to Occam's razor as, according to such a heuristic, simpler explanations are preferred to more complicated ones. Only in situations where extraordinary evidence exists would an extraordinary claim be the simplest explanation. It appears in hypothesis testing, where the hypothesis that there is no evidence for the proposed phenomenon, what is known as the "null hypothesis", is preferred. The formal argument involves assigning a stronger Bayesian prior to the acceptance of the null hypothesis as opposed to its rejection.
== Origin and precursors ==
Science communicator Carl Sagan popularized the aphorism in his 1979 book Broca's Brain, and in his 1980 television show Cosmos in reference to claims about extraterrestrials visiting Earth. Sagan had first stated the eponymous standard in a 1977 interview with The Washington Post. However, scientific skeptic Marcello Truzzi used the formulation "extraordinary claims require extraordinary proof" in an article published by Parapsychology Review in 1975, as well as in a Zetetic Scholar article in 1978. Two 1978 articles quoted physicist Philip Abelson—then the editor of the journal Science—using the same phrasing as Truzzi.
In his 1748 essay "Of Miracles", philosopher David Hume wrote that if "the fact ... partakes of the extraordinary and the marvellous ... the evidence ... received a diminution, greater or less, in proportion as the fact is more or less unusual". Deming concluded that this was the first complete elucidation of the standard. Unlike Sagan, Hume defined the nature of "extraordinary": he wrote that it was a large magnitude of evidence.
Others had also put forward very similar ideas. Quote Investigator cites similar statements from Benjamin Bayly (in 1708), Arthur Ashley Sykes (1740), Beilby Porteus (1800), Elihu Palmer (1804), and William Craig Brownlee (1824). The French scholar Pierre-Simon Laplace, in essays (1810 and 1814) on the stability of the Solar System, wrote that "the weight of evidence for an extraordinary claim must be proportioned to its strangeness". Thomas Jefferson in an 1808 letter expressed contemporary skepticism of meteorites thus: "A thousand phenomena present themselves daily which we cannot explain, but where facts are suggested, bearing no analogy with the laws of nature as yet known to us, their verity needs proofs proportioned to their difficulty."
== Analysis and criticism ==
Sagan did not describe any concrete or quantitative parameters as to what constitutes "extraordinary evidence", which raises the issue of whether the standard can be applied objectively. Academic and climate-change denialist David Deming notes that it would be "impossible to base all rational thought and scientific methodology on an aphorism whose meaning is entirely subjective". He instead argues that "extraordinary evidence" should be regarded as a sufficient amount of evidence rather than evidence deemed of extraordinary quality. Tressoldi noted that the threshold of evidence is typically decided through consensus. This problem is less apparent in clinical medicine and psychology, where statistical results can establish the strength of evidence.
Deming also noted that the standard can "suppress innovation and maintain orthodoxy". Others, like Etzel Cardeña, have noted that many scientific discoveries that spurred paradigm shifts were initially deemed "extraordinary" and likely would not have been so widely accepted if extraordinary evidence were required. Uniform rejection of extraordinary claims could affirm confirmation biases in subfields. Additionally, there are concerns that, when inconsistently applied, the standard exacerbates racial and gender biases. Psychologist Richard Shiffrin has argued that the standard should not be used to bar research from publication but to ascertain what is the best explanation for a phenomenon. Conversely, mathematical psychologist Eric-Jan Wagenmakers stated that extraordinary claims are often false and their publication "pollutes the literature". To qualify the publication of such claims, psychologist Suyog Chandramouli has suggested the inclusion of peer reviewers' opinions on their plausibility or an attached curation of post-publication peer evaluations.
Cognitive scientist and AI researcher Ben Goertzel believes that the phrase is used as a "rhetorical meme" without critical thought. Philosopher Theodore Schick argued that "extraordinary claims do not require extraordinary evidence" if they provide the most adequate explanation. Moreover, theists and Christian apologists like William Lane Craig have argued that it is unfair to apply the standard to religious miracles, as other improbable claims are often accepted based on limited testimonial evidence, such as an individual claiming that they won the lottery.
== See also ==
Epistemology
Hitchens's razor
Logical positivism
Philosophical razor
Theory of justification
Hanlon's razor
== References ==
=== Citations ===
=== Works cited ===
==== Books ====
==== Journal articles ====
==== Other media ====
== External links ==

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The law of hyperbolic growth of the human population is an empirical law discovered by Heinz von Foerster, which states that the human population of the Earth has grown hyperbolically over several millennia. In the article published by Foerster et al. it was noted that hyperbolic growth is possible only if humanity acts "as a single player", that is, under the condition of some form of cooperation among all people on Earth. Most authors explain the hyperbolic growth by the joint intellectual development of humanity. At the same time, many (S. Kuznets, J. Simon, M. Kremer, S. V. Tsirel, A. V. Korotayev and others) consider the development of technologies as the main factor. A. V. Podlazov highlights life-saving technologies, which are understood not only as production methods but "in general, any knowledge and skills that can be used to save a person from death or prolong their life". S. P. Kapitsa and a number of other authors name the accumulation of knowledge and information in general as the cause of growth.
== Boundaries of the law's application ==
According to statistical data, the law of hyperbolic growth ceased to operate in the 1960–1970s. Since 1989, the absolute rates of world population growth have also begun to decline, so it is no longer possible to speak even of linear population growth. According to the model of the French physician Jean-Noël Biraben, the growth limit will be 10–12 billion people; most other models suggest fairly close levels of world population stabilization. Quite plausible are also scenarios of a decrease in the Earth's population after reaching its maximum value.
Various views have been expressed regarding the beginning of the hyperbolic law's action. In the work of Heinz von Foerster, it was shown that the law of hyperbolic growth has been in effect since the beginning of the Common Era. Astrophysicist Sebastian von Hoerner believed that the hyperbolic law operated throughout the existence of humanity. S. P. Kapitsa, based on the model he developed, calculated the date of the law's beginning as 1.6 million years ago. Other authors usually limit themselves to the period for which there are more or less reliable empirical estimates, for example 40 or 10 thousand years.
Although the general hyperbolic nature of demographic dynamics is not in doubt, a careful analysis of empirical data shows that the parameters of the hyperbola were not constant. In particular, before the beginning of the Common Era (5th–1st millennium BC), the growth rate was higher than later. A significant change in parameters in the 1st millennium AD is masked by the explosive population growth in recent centuries, compared to which all the vicissitudes of previous history seem insignificant.
== Mathematical formulations ==
The law received its name because the dynamics of the Earth's population approximately corresponds to a hyperbola – a second-order mathematical curve:
N
(
t
)
=
C
t
0
−
t
.
{\displaystyle N(t)={\frac {C}{t_{0}-t}}.}
Here
N
(
t
)
{\displaystyle N(t)}
is the world population in year
t
{\displaystyle t}
,
t
0
{\displaystyle t_{0}}
is the so-called singularity, the point in time when the world population would become infinite if hyperbolic growth continued (2025, according to von Hoerner's calculations),
C
{\displaystyle C}
is a constant; for von Hoerner, 200 billion person-years. Hyperbolic growth is most clearly manifested through doublings: each subsequent doubling of humanity's population occurred approximately twice as fast as the previous one. This can be especially clearly observed in the interval 1650–1970.
The law can also be represented in differential form:
d
N
d
t
=
N
2
C
,
{\displaystyle {\frac {dN}{dt}}={\frac {N^{2}}{C}},}
that is, the population growth rate is proportional to the square of the current population. Since these equations correspond to unlimited growth at the singularity point, a number of authors, starting with M. Kremer and S. P. Kapitsa, build models describing the deviation from this singularity, which has actually been occurring since the 1960–1970s.
== Technological justification of hyperbolic growth ==
M. Kremer proposed a rigorous mathematical justification for hyperbolic growth, based on the assumptions that population size is proportional to the level of technological development, and the rate of technological development, in turn, depends on the number of "inventors", which is proportional to the population size. Most models of human population growth developed recently are based on Kremer's equation (for example and others). The model of Korotayev–Malkov–Khalturina stands out especially, which also includes Kremer's equation. Without claiming to describe the entire demographic history of humanity, it very well describes the growth dynamics on the stages of 5000 BC–500 AD and 500–2025 (forecast) years.
In the theory of S. Kuznets–M. Kremer, the literal understanding that in any era per thousand people there is supposedly a constant number of "standard inventors" with unchanging efficiency in improving technologies is criticized. In particular, because "in fact, the vast majority of inventions were obtained in individual, often small, countries in special eras (ancient Greece, Song China, Italy of the Renaissance era, England during the Industrial Revolution and others), while huge regions of the world invented very little" (S. V. Tsirel).
== Life-saving technologies ==
The highlighting of life-saving technologies, proposed by A. V. Podlazov, has the meaning that skills and knowledge contributing to people's survival spread the fastest. In times when humanity was divided by insurmountable distances and communications between peoples were not regular, only such, the most relevant for everyone, information could spread at a sufficient speed for that time. A. V. Podlazov also developed a model that very well describes the dynamics of human population growth.

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== Accumulation of information ==
In the works of S. P. Kapitsa the independence of human development from available resources is substantiated. Based on this position, the principle of demographic imperative is advanced, as the self-sufficiency of demography in describing human history. At the same time, leading importance in the cooperative nonlinear mechanism of development is given to the informational interaction of large groups of people. It is the accumulation of information in the process of such interaction that can explain the hyperbolic growth of the human population. Information has a more fundamental character than the technological level and differs from it in integrity: any information can be in demand for creating new technologies, whereas the state of humanity cannot be described by limiting to used technologies.
According to Kapitsa, humanity is near the inflection point of the population growth curve, falling around 2005. After passing this point, a slowdown was expected, symmetric to the era of hyperbolic growth. Kapitsa's works are criticized for excessive physicalism.
The accumulation of information and the associated hyperbolic growth of species diversity was also noted until recently (before human intervention) in the biosphere.
The widely discussed opinion is that further civilization development will be associated precisely with the growth of the volume of information in the human-machine supermind (co-intelligence, synergistic intelligence), possibly based on the Internet. A person can enter the supermind simply as an Internet user, or by improving their biological nature, as a cyborg.
== See also ==
Exponential growth
Population boom
Demographic transition
Doomsday argument
== Notes ==
== References ==
Korotayev, A. (2007). "Compact Mathematical Models of World System Development, and How they can Help us to Clarify our Understanding of Globalization Processes". In Modelski, George; Devezas, Tessaleno; Thompson, William R. (eds.). Globalization as Evolutionary Process: Modeling Global Change. London: Routledge. pp. 133–160.
Ozhovan, M.; Loschinin, M. (2015). "Heuristic Paradoxes of S. P. Kapitsa's Theoretical Demography" (PDF). European Researcher (92 (3)): 237–248.

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The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields pertaining to geological stratigraphy. In its plainest form, it states that in undeformed stratigraphic sequences, the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface to form new deposits over time. This is paramount to stratigraphic dating, which requires a set of assumptions, including that the law of superposition holds true and that an object cannot be older than the materials of which it is composed. To illustrate the practical applications of superposition in scientific inquiry, sedimentary rock that has not been deformed by more than 90° will exhibit the oldest layers on the bottom, thus enabling paleontologists and paleobotanists to identify the relative ages of any fossils found within the strata, with the remains of the most archaic lifeforms confined to the lowest. These findings can inform the community on the fossil record covering the relevant strata, to determine which species coexisted temporally and which species existed successively in perhaps an evolutionarily or phylogenetically relevant way.
== History ==
The law of superposition was first proposed in 1669 by the Danish scientist Nicolas Steno, and is present as one of his major theses in the groundbreaking seminal work Dissertationis prodromus (1669).
In the English-language literature, the law was popularized by William "Strata" Smith, who used it to produce the first geologic map of Britain. It is the first of Smith's laws, which were formally published in Strata Identified by Fossils (1816–1819).
== Archaeological considerations ==
Superposition in archaeology and especially in stratification use during excavation is slightly different as the processes involved in laying down archaeological strata are somewhat different from geological processes. Human-made intrusions and activity in the archaeological record need not form chronologically from top to bottom or be deformed from the horizontal as natural strata are by equivalent processes. Some archaeological strata (often termed as contexts or layers) are created by undercutting previous strata. An example would be that the silt back-fill of an underground drain would form some time after the ground immediately above it. Other examples of non vertical superposition would be modifications to standing structures such as the creation of new doors and windows in a wall. Superposition in archaeology requires a degree of interpretation to correctly identify chronological sequences and in this sense superposition in archaeology is more dynamic and multi-dimensional.
== Other limitations to stratification and superposition ==
Original stratification induced by natural processes can subsequently be disrupted or permutated by a number of factors, including animal interference and vegetation, as well as limestone crystallization.
Stratification behaves in a different manner with surface-formed igneous depositions, such as lava flows and ash falls, and thus superposition may not always successfully apply under certain conditions.
== See also ==
Harris matrix
Principle of cross-cutting relationships
Principle of faunal succession
Principle of lateral continuity
Principle of original horizontality
Stratification (archeology)
Stratigraphy
Structural geology
== References ==
=== General sources ===
Hamblin, W.K. The Earth's Dynamic Systems: A Textbook in Physical Geology, by W. Kenneth Hamblin, BYU, Provo, UT, Illus. William L. Chesser, Dennis Tasa, (Burgess Publishing Company, Minneapolis, Minnesota), c. 1978, p. 115, "The Principle of Superposition and Original Horizontality;" p. 116: The Law of Faunal Succession, "The Principle of Crosscutting Relations;" pp. 116-17: "The Principle of Inclusion," (as in the Steno discussion above).
Principles of Archaeological Stratigraphy. 40 figs. 1 pl. 136 pp. London & New York: Academic Press ISBN 0-12-326650-5

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Marine isotope stages (MIS), marine oxygen-isotope stages, or oxygen isotope stages (OIS), are alternating warm and cool periods in the Earth's paleoclimate, deduced from oxygen isotope data derived from deep sea core samples. Working backwards from the present, which is MIS 1 in the scale, stages with even numbers have high levels of oxygen-18 and represent cold glacial periods, while the odd-numbered stages are lows in the oxygen-18 figures, representing warm interglacial intervals. The data are derived from pollen and foraminifera (plankton) remains in drilled marine sediment cores, sapropels, and other data that reflect historic climate; these are called proxies.
The MIS timescale was developed from the pioneering work of Cesare Emiliani in the 1950s, modifying an earlier system introduced by oceanographer Gustaf Arrhenius. It is now widely used in archaeology and other fields to express dating in the Quaternary period (the last 2.6 million years), as well as providing the fullest and best data for that period for paleoclimatology or the study of the early climate of the Earth, representing "the standard to which we correlate other Quaternary climate records". Emiliani's work in turn depended on Harold Urey's prediction in a paper of 1947 that the ratio between oxygen-18 and oxygen-16 isotopes in calcite, the main chemical component of the shells and other hard parts of a wide range of marine organisms, should vary depending on the prevailing water temperature in which the calcite was formed.
Over 100 stages have been identified, currently going back some 6 million years, and the scale may eventually reach back as far as 15 million years. Some stages, in particular MIS 5, are divided into sub-stages, such as "MIS 5a", with 5 a, c, and e being warm and b and d cold. A numeric system for referring to "horizons" (events rather than periods) may also be used, with for example MIS 5.5 representing the peak point of MIS 5e, and 5.51, 5.52 etc. representing the peaks and troughs of the record at a still more detailed level. For more recent periods, increasingly precise resolution of timing continues to be developed.
== Developing a timescale ==
In 1957 Emiliani moved to the University of Miami to have access to core-drilling ships and equipment, and began to drill in the Caribbean and collect core data. A further important advance came in 1967, when Nicholas Shackleton suggested that the fluctuations over time in the marine isotope ratios that had become evident by then were caused not so much by changes in water temperature, as Emiliani thought, but mainly by changes in the volume of ice-sheets, which when they expanded took up the lighter oxygen-16 isotope in preference to the heavier oxygen-18. The cycles in the isotope ratio were found to correspond to terrestrial evidence of glacials and interglacials. A graph of the entire series of stages then revealed unsuspected advances and retreats of ice and also filled in the details of the stadials and interstadials.
More recent ice core samples of today's glacial ice substantiated the cycles through studies of ancient pollen deposition. Currently a number of methods are making additional detail possible. Matching the stages to named periods proceeds as new dates are discovered and new regions are explored geologically. The marine isotopic records appear more complete and detailed than any terrestrial equivalents, and have enabled a timeline of glaciation for the Plio-Pleistocene to be identified. It is now believed that changes in the size of the major ice sheets such as the historical Laurentide Ice Sheet of North America are the main factor governing variations in the oxygen isotope ratios.
The MIS data also matches the astronomical data of Milankovitch cycles of orbital forcing or the effects of variations in insolation caused by cyclical slight changes in the tilt of the Earth's axis of rotation – the "orbital theory". Indeed, that the MIS data matched Milankovich's theory, which he formed during World War I, so well was a key factor in the theory gaining general acceptance, despite some remaining problems at certain points, notably the so-called 100,000-year problem. For relatively recent periods data from radiocarbon dating and dendrochronology also support the MIS data. The sediments also acquire depositional remanent magnetization which allows them to be correlated with earth's geomagnetic reversals. For older core samples, individual annual depositions cannot usually be distinguished, and dating is taken from the geomagnetic information in the cores. Other information, especially as to the ratios of gases such as carbon dioxide in the atmosphere, is provided by analysis of ice cores.
The SPECMAP Project, funded by the US National Science Foundation, has produced one standard chronology for oxygen isotope records, although there are others. This high resolution chronology was derived from several isotopic records, the composite curve was then smoothed, filtered and tuned to the known cycles of the astronomical variables. The use of a number of isotopic profiles was designed to eliminate 'noise' errors, that could have been contained within a single isotopic record. Another large research project funded by the US government in the 1970s and 1980s was Climate: Long range Investigation, Mapping, and Prediction (CLIMAP), which to a large degree succeeded in its aim of producing a map of the global climate at the Last Glacial Maximum, some 18,000 years ago, with some of the research also directed at the climate some 120,000 years ago, during the last interglacial. The theoretical advances and greatly improved data available by the 1970s enabled a "grand synthesis" to be made, best known from the 1976 paper Variations in the earth’s orbit: pacemaker of the ice ages (in Science), by J.D. Hays, Shackleton and John Imbrie, which is still widely accepted, and covers the MIS timescale and the causal effect of the orbital theory.
In 2010 the Subcommission on Quaternary Stratigraphy of the International Commission on Stratigraphy dropped other lists of MIS dates and started using the Lisiecki & Raymo (2005) LR04 Benthic Stack, as updated. This was compiled by Lorraine Lisiecki and Maureen Raymo.
== Stages ==

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