Spaced Repetition

The spacing effect is a well-documented cognitive phenomenon in which information studied across multiple distributed sessions is retained better than information studied in a single massed session. Research on the effect dates back to Hermann Ebbinghaus's memory experiments published in 1885.

Popular spaced repetition tools include Anki, SuperMemo, Quizlet, and Memrise, which schedule flashcard reviews based on how well the user recalls each item. Anki, one of the most widely used, employs an algorithm derived from SuperMemo's SM-2 algorithm, originally developed by Piotr Woźniak in the late 1980s.

In a typical system, correctly recalling an item increases the interval before its next review (e.g., from 1 day to 3 days to 7 days to 21 days), while an incorrect recall resets the interval to a short value. This targets reviews to occur just before the learner is likely to forget, based on the forgetting curve.

Research consistently shows that spaced repetition improves long-term retention of vocabulary and factual material compared to cramming or unstructured review. Multiple studies in applied linguistics and educational psychology have demonstrated significant benefits for second-language vocabulary acquisition over periods of weeks and months.