How spaced repetition works, and why it beats cramming

The forgetting curve

In the 1880s, the psychologist Hermann Ebbinghaus studied how quickly he forgot lists of nonsense syllables. He found that forgetting is fast at first, then slows down, and that reviewing a memory makes it last longer.

Later research on the spacing effect found the same thing in many settings: the same amount of study, spread over several days, tends to be remembered better than the same study done in one sitting.

How a review schedule works

A spaced repetition system shows you each card, asks you to rate how well you remembered it, and picks the next review date from your answer. If you remembered easily, the gap grows. If you forgot, the card comes back soon.

A typical card might return after a day, then three days, then a week, then a few weeks. Cards you find hard come back more often, and cards you know well almost disappear.

• Again: you forgot it, so it returns soon.
• Hard: you got it with effort, so the gap grows only a little.
• Good: the normal case, so the gap grows.
• Easy: you knew it instantly, so the gap grows a lot.

Why cramming feels better but works worse

Cramming works for tomorrow's quiz, because the material is fresh in your mind. The trouble is that it fades quickly, so you relearn it from scratch for the final exam. Spaced reviews take less time in total, because each one only has to top up a memory that is still partly there.

What it is good for, and what it is not

Spaced repetition is excellent for facts, vocabulary, definitions, formulas and anything you need to recall on demand. It is not a substitute for understanding. For ideas that need explaining, such as why a proof works, combine it with writing out explanations and solving problems.

Getting started

Make cards from your own notes, keep each card to one idea, and review a little every day instead of a lot once a week. Ten minutes a day is a good start. Missing a day is fine: overdue cards simply wait for you.