What is spaced repetition?

The idea in one example

You learn a new word today. Instead of reviewing it ten times this evening, you check it again in a few days. If you still know it, the next check is a week later, then two weeks, then a month. If you have forgotten it, the gap shrinks and you see it sooner.

Review	If you remembered	If you forgot
1	Next in about 3 days	Back soon, meaning shown again
2	Next in about a week	Gap shortened
3	Next in about two weeks	Gap shortened
4	Next in about a month	Gap shortened

The exact intervals vary between systems. The principle - widen after success, shorten after failure - is the same everywhere.

Why it works

Two effects combine. The spacing effect: material reviewed across separate sessions is remembered longer than the same amount of review crammed into one. And the testing effect: trying to recall something strengthens memory more than rereading it.

A review that comes a little later, when recall takes some effort, does more than an easy one. That is why the gaps widen - each successful recall buys a longer wait before the next one is needed.

Where it came from

The observations go back to Hermann Ebbinghaus, who measured forgetting in the 1880s. In the 1970s the German science writer Sebastian Leitner popularised a paper version using flashcards moved between boxes. In the 1980s Piotr Woźniak turned the idea into software with SuperMemo, whose SM-2 scheduling algorithm still underlies many flashcard apps today.

Common misunderstandings

• It is not about reviewing often. It is about reviewing at the right time, which usually means less often than you would expect.
• Getting a card wrong is not a failure. It is information the schedule needs.
• It does not replace understanding. Spaced repetition keeps what you have learned; it works best on things you already understand the first time.