How SM-2 Spaced Repetition Works

In the 1880s, psychologist Hermann Ebbinghaus measured how quickly we forget new information and found a pattern: memory decays fast at first, then flattens out. Without review, most of what you learn today is gone within days.

The useful part of the curve is its shape. Right before a memory fades is the perfect moment to review it — the recall just barely succeeds, and that effort is exactly what strengthens the trace. Review too early and you waste time on something you already knew; review too late and you relearn from scratch.

SM-2, published by Piotr Woźniak in 1987 as the engine of SuperMemo, turns this idea into a concrete schedule. Every card carries two numbers: how long until its next review (the interval) and how easy the item has proven for you (the ease factor, starting at 2.5).

The first successful review schedules the card for tomorrow. The second jumps to six days. After that, each new interval is the previous one multiplied by the ease factor — so a word you find easy spreads out quickly, while a stubborn one keeps coming back sooner.

After each review you grade how well you remembered. Good grades raise the ease factor and stretch future intervals; a failure resets the interval to one day and lowers the factor, so difficult material gets drilled instead of drifting away. In Vocabie this is the one-tap feedback after each review — a modern adaptation of SM-2's original 0–5 grading.

Vocabulary is the ideal fit for spaced repetition: items are small, independent, and needed for the long term. Capturing a word while you read hands SM-2 a card at exactly the moment of curiosity; the algorithm then makes sure it resurfaces at the edge of forgetting until it sticks for good.

That loop — capture while reading, review before forgetting — is what Vocabie is built around.