Spaced Repetition

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Spaced repetition is an evidence-based learning technique that is usually performed with flashcards.

Newly introduced and more difficult flashcards are shown more frequently, while older and less difficult flashcards are shown less frequently in order to exploit the psychological spacing effect.

Evidence-Based Education

Evidence-based education is the principle that education practices should be based on the best available scientific evidence, rather than tradition, personal judgement, or other influences.

Research has shown that spaced repetiion "leads to more robust memory formation than does massed training, which involves short or no intervals".

Spacing Effect

Spacing effect demonstrates that learning is more effective when study sessions are spaced out. This effect shows that more information is encoded into long-term memory by spaced study sessions, than by massed presentation ("cramming").


There are many programatic implementations for scheduling spaced repetition:

  • Neural network based
  • Leitner system: 5 levels and an arbitrary number of stages
  • SM-family of algorithms (SuperMemo): SM-0 (a paper implementation) to SM-18 (SuperMemo 18)


List of Software

Leitner System

The Leitner system is a paper based implementation 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 the 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 learn is required to revisit the cards.

In Leitner's original method, published in his book So lernt man Lemen (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.