Spaced repetition is a method of reviewing material at expanding time intervals, timed to coincide with the moment when the learner is on the verge of forgetting. In language study, it is most commonly applied to vocabulary acquisition.
The Origin of the SM-2 Algorithm
The SuperMemo 2 (SM-2) algorithm was developed in 1987 by Piotr Woźniak, a Polish researcher and cognitive scientist, as part of his work on optimising memory retention. Woźniak's key insight was that the ideal time to review a piece of information is not fixed — it expands as the memory becomes more consolidated. Reviewing too early wastes study time; reviewing too late allows the memory trace to decay.
The algorithm assigns each flashcard an interval (measured in days) and an easiness factor (EF). After each review, the learner rates their recall quality on a scale of 0 to 5. The EF is adjusted accordingly, and the next interval is calculated. Cards rated poorly receive short intervals; cards rated well receive progressively longer ones.
How Anki Implements Spaced Repetition
Anki is an open-source flashcard application that implements a modified version of the SM-2 algorithm. It remains among the most widely used tools for vocabulary study, particularly in medical education and language learning communities. Cards are presented in daily review sessions. The app tracks performance across sessions and schedules reviews automatically.
For language learners in Poland, Anki decks covering Polish vocabulary, German grammar forms, and English phrasal verbs are widely shared through the AnkiWeb shared decks platform.
Practical Considerations for Adult Learners
Several aspects of SRS are particularly relevant for adult learners:
- Session size: Reviews accumulate quickly if study is skipped. Adults with irregular schedules often find it more sustainable to review fewer cards per day (20–50) rather than attempting large backlogs.
- Card quality: Single-word front / single-word back cards are generally less effective than sentence-context cards. Seeing a word in context improves encoding.
- Integration with other input: SRS works best alongside listening and reading input. Encountering a word in a film or book reinforces what was reviewed in Anki, and vice versa.
Alternative SRS Implementations
Several other tools implement SRS principles:
- SuperMemo: Woźniak's original software, still maintained and available at supermemo.com. More complex than Anki, with a steeper learning curve.
- Duolingo: Incorporates SRS elements alongside gamified content. Less customisable than dedicated flashcard tools.
- Memrise: Community-curated decks with audio and image support, implementing interval repetition.
Limitations of SRS in Isolation
Spaced repetition addresses vocabulary retention but does not develop grammar, listening comprehension, or speaking fluency directly. Researchers in second language acquisition consistently note that SRS is most effective when combined with substantial amounts of comprehensible input. The ability to recognise a word in isolation differs from the ability to use it accurately in production.
For learners at B1 level and above, grammar exposure through reading and structured output practice typically becomes the primary bottleneck — at which point SRS continues to serve as a maintenance tool for vocabulary rather than the primary driver of progress.