---
title: "Hierarchy of evidence"
chunk: 2/3
source: "https://en.wikipedia.org/wiki/Hierarchy_of_evidence"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:25:58.212894+00:00"
instance: "kb-cron"
---

=== Khan et al. ===
A protocol for evaluation of research quality was suggested by a report from the Centre for Reviews and Dissemination, prepared by Khan et al. and intended as a general method for assessing both medical and psychosocial interventions. While strongly encouraging the use of randomized designs, this protocol noted that such designs were useful only if they met demanding criteria, such as true randomization and concealment of the assigned treatment group from the client and from others, including the individuals assessing the outcome. The Khan et al. protocol emphasized the need to make comparisons on the basis of "intention to treat" in order to avoid problems related to greater attrition in one group. The Khan et al. protocol also presented demanding criteria for nonrandomized studies, including matching of groups on potential confounding variables and adequate descriptions of groups and treatments at every stage, and concealment of treatment choice from persons assessing the outcomes. This protocol did not provide a classification of levels of evidence, but included or excluded treatments from classification as evidence-based depending on whether the research met the stated standards.

=== U.S. National Registry of Evidence-Based Practices and Programs ===
An assessment protocol has been developed by the U.S. National Registry of Evidence-Based Practices and Programs (NREPP). Evaluation under this protocol occurs only if an intervention has already had one or more positive outcomes, with a probability of less than .05, reported, if these have been published in a peer-reviewed journal or an evaluation report, and if documentation such as training materials has been made available. The NREPP evaluation, which assigns quality ratings from 0 to 4 to certain criteria, examines reliability and validity of outcome measures used in the research, evidence for intervention fidelity (predictable use of the treatment in the same way every time), levels of missing data and attrition, potential confounding variables, and the appropriateness of statistical handling, including sample size.

== History ==

=== Canada ===
The term was first used in a 1979 report by the "Canadian Task Force on the Periodic Health Examination" (CTF) to "grade the effectiveness of an intervention according to the quality of evidence obtained".
The task force used three levels, subdividing level II:

Level I: Evidence from at least one randomized controlled trial,
Level II1: Evidence from at least one well designed cohort study or case control study, preferably from more than one center or research group.
Level II2: Comparisons between times and places with or without the intervention
Level III: Opinions of respected authorities, based on clinical experience, descriptive studies or reports of expert committees.
The CTF graded their recommendations into a 5-point A–E scale: A: Good level of evidence for the recommendation to consider a condition, B: Fair level of evidence for the recommendation to consider a condition, C: Poor level of evidence for the recommendation to consider a condition, D: Fair level evidence for the recommendation to exclude the condition, and E: Good level of evidence for the recommendation to exclude condition from consideration.
The CTF updated their report in 1984, in 1986 and 1987.

=== United States ===
In 1988, the United States Preventive Services Task Force (USPSTF) came out with its guidelines based on the CTF using the same three levels, further subdividing level II.

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 or case-control analytic 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.
Over the years many more grading systems have been described.

=== United Kingdom ===
In September 2000, the Oxford (UK) Centre for Evidence-Based Medicine (CEBM) Levels of Evidence published its guidelines for 'Levels' of evidence regarding claims about prognosis, diagnosis, treatment benefits, treatment harms, and screening. It not only addressed therapy and prevention, but also diagnostic tests, prognostic markers, or harm. The original CEBM Levels was first released for Evidence-Based On Call to make the process of finding evidence feasible and its results explicit. As published in 2009 they are:

1a: Systematic reviews (with homogeneity) of randomized controlled trials
1b: Individual randomized controlled trials (with narrow confidence interval)
1c: All or none (when all patients died before the treatment became available, but some now survive on it; or when some patients died before the treatment became available, but none now die on it.)
2a: Systematic reviews (with homogeneity) of cohort studies
2b: Individual cohort study or low quality randomized controlled trials (e.g. <80% follow-up)
2c: "Outcomes" Research; ecological studies
3a: Systematic review (with homogeneity) of case-control studies
3b: Individual case-control study
4: Case series (and poor quality cohort and case-control studies)
5: Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles"
In 2011, an international team redesigned the Oxford CEBM Levels to make it more understandable and to take into account recent developments in evidence ranking schemes. The Levels have been used by patients, clinicians and also to develop clinical guidelines including 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.

=== Global ===
In 2007, the World Cancer Research Fund grading system described 4 levels: Convincing, probable, possible and insufficient evidence. All Global Burden of Disease Studies have used it to evaluate epidemiologic evidence supporting causal relationships.

== Proponents ==
In 1995 Wilson et al., in 1996 Hadorn et al. and in 1996 Atkins et al. have described and defended various types of grading systems.