SM-2 — the algorithm behind Anki and most spaced repetition tools — was published in 1987 and is remarkably durable. It is also wrong in specific, predictable ways that matter once your deck grows. What SM-2 does Each card carries an ease factor, starting at 2.5. Grade a review 0-5; on success the interval multiplies by ease, on failure it resets: if quality >= 3 : interval = 1 if reps == 0 else 6 if reps == 1 else round ( in...