how spaced repetition actually works: the sm-2 algorithm

most people study inefficiently. we review things too early - wasting time. or too late - after we've already forgotten them. the real question is simple: when is the optimal moment to review something? in 1987, a polish researcher named piotr woźniak proposed a surprisingly simple answer. he built an algorithm that schedules reviews so that you see information just before you forget it . that algorithm is called sm-2 , and variations of it power spaced-repetition tools used by millions of people today, including systems like anki. the fascinating part is that the core idea is extremely small. the entire algorithm fits in a few variables and a short formula. but to understand why it works, we first need to look at how memory behaves. human memory decays quickly. in the late 19th century, psychologist hermann ebbinghaus studied how quickly people forget newly learned information. his experiments produced what we now call the "forgetting curve" - a model showing that memory retention dro