SM-2 Spaced Repetition Algorithm Has Predictable Flaws That Worsen at Scale

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SM-2, the algorithm powering Anki and most spaced repetition tools, was developed in 1987 and remains widely used despite several structural limitations. Its ease-factor system acts as a one-way ratchet, permanently penalising cards that were failed early even after a learner has since mastered them. The algorithm also ignores response latency, treats every card in isolation without accounting for how related or confusable items interact, and applies a fixed memory-decay curve regardless of individual learners or material type. A newer alternative called FSRS models memory using three variables — difficulty, stability, and retrievability — and adapts to a user's review history to allow explicit retention targets. Experts note that SM-2's shortcomings are largely invisible in small decks of around 200 cards but become increasingly problematic as decks grow beyond 5,000 cards.

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