---
title: "Spaced repetition"
chunk: 3/3
source: "https://en.wikipedia.org/wiki/Spaced_repetition"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:26:29.414379+00:00"
instance: "kb-cron"
---

Leitner system—a simple scheme that uses five levels and an arbitrary number of study stages
Neural-network-based
The SM family of algorithms (SuperMemo#Algorithms), ranging from SM-0 (a paper-and-pencil prototype) to SM-18, which is built into SuperMemo 18 and 19.
The DASH (Difficulty, Ability and Study History) family
SSP-MMC (Stochastic Shortest Path Minimize Memorization Cost) and the closely related FSRS (Free Spaced Repetition Scheduler), which is available in Anki starting with release 23.10 and in RemNote starting with release 1.16

== Evidence and criticism ==
Spaced repetition is widely accepted as a performant learning strategy in a number of domains, with many researchers suggesting implementing this method in formal education. There is evidence that the popular method of "expanding intervals" (when the interval between the repetitions increases with each repetition) performs as well as or better than uniformly spaced repetitions. Some papers find expanding intervals to be beneficial for recall. Other meta-analyses tend to conclude that both methods yield similar results, therefore concluding that "strong recommendations to teachers and students in favor of spaced retrieval practice are warranted". 
Several mechanisms were suggested for expanding intervals providing an additional benefit; the most notable one is that one of the core tenets of spaced repetition is that spacing increases the effort for retrieval, and that expanding intervals allow to gradually increase that difficulty. However, little evidence has been found to back this claim. It has been argued that the benefit observed for expanding intervals in some studies is due to other factors, such as the timing of the first retrieval, the number of repetitions or the overall spacing between the tests. It has also been proposed that the best schedule is learner-dependent, making general recommendations irrelevant.

== Implementations ==

=== Software ===

Most spaced repetition software (SRS) is modeled after the manual style of learning with physical flashcards: items to memorize are entered into the program as question-answer pairs. When a pair is due to be reviewed, the question is displayed on a screen, and the user must attempt to answer. After answering, the user manually reveals the answer and then tells the program (subjectively) how difficult answering was. The program schedules pairs based on spaced repetition algorithms. Without a computer program, the user has to schedule physical flashcards; this is time-intensive and limits users to simple algorithms like the Leitner system.
To optimize review schedules, developments in spaced repetition algorithms focus on predictive modeling. These algorithms use randomly determined equations to determine the most effective timing for review sessions.
Further refinements with regard to software:

Confidence-based repetition: A user rates their confidence in each digital flashcard, e.g. on a scale of 1–5; a lower-confidence card is repeated more frequently until the user upgrades their confidence rating in it.
Questions and/or answers can be a sound file to train recognition of spoken words.
Automatic generation of pairs (e.g. for vocabulary, it is useful to generate three question-pairs: written foreign word, its pronunciation and its meaning, but data only has to be entered once.)
Additional information retrieved automatically is available, such as example sentences containing a word.
Opportunities to combine spaced repetition with online community functions, e.g. sharing courses.

=== Paper flash cards ===

The Leitner system is a widely used method of efficiently using flashcards that was proposed by the German science journalist Sebastian Leitner in the 1970s. It is a simple implementation of the principle of spaced repetition, where cards are reviewed at increasing intervals.
In this method, flashcards are sorted into groups according to how well the learner knows each one in Leitner's learning box. The learners try to recall the solution written on a flashcard. If they succeed, they send the card to the next group. If they fail, they send it back to the first group. Each succeeding group has a longer period of time before the learner is required to revisit the cards. In Leitner's original method, published in his book So lernt man Lernen (How To Learn To Learn), the schedule of repetition was governed by the size of the partitions in the learning box. These were 1, 2, 5, 8 and 14 cm. Only when a partition became full was the learner to review some of the cards it contained, moving them forward or back, depending on whether they remembered them.

=== Audio instruction ===
Graduated-interval recall is a type of spaced repetition published by Paul Pimsleur in 1967.  It is used in the Pimsleur language learning system and is particularly suited to programmed audio instruction due to the very short times (measured in seconds or minutes) between the first few repetitions, as compared to other forms of spaced repetition which may not require such precise timings. The intervals published in Pimsleur's paper were: 5 seconds, 25 seconds, 2 minutes, 10 minutes, 1 hour, 5 hours, 1 day, 5 days, 25 days, 4 months, and 2 years.

== References ==

== Further reading ==
Kail, R. V., & Cavanaugh J. C. (2007). "Spaced Retrieval". Human Development: A Life-Span View (5th ed.). Belmont, CA: Wadsworth.
Wozniak, Piotr (February 1999). "Effective learning: Twenty rules of formulating knowledge". – advice on making flashcards for spaced repetition.