What spaced repetition is, and why it works

Reviewing something at the moment you are about to forget it is the most reliable way to keep it. Here is what that means in practice, and where it usually goes wrong.

Spaced repetition is the practice of reviewing something at increasing intervals: a few days after you first learn it, then a week or so later, then a month, then several months. Each successful recall buys you a longer gap before the next one.

It is not a study technique in the motivational sense. It is closer to a schedule. The decisions about what to review and when are handed over to a system, so the only thing left for you to do is the reviewing.

Why waiting until you have nearly forgotten works better

The finding it rests on is old and unusually well replicated. In the 1880s Hermann Ebbinghaus tested himself on nonsense syllables over months and plotted how fast he lost them. The shape he found, steep at first and then flattening, is now called the forgetting curve, and the useful part is what happens when you interrupt it: each review makes the next stretch of the curve shallower, so the same material survives longer with less work.

The second finding matters more and is less well known. Recalling something from memory strengthens it far more than reading it again does. This is the testing effect, and it is why a flashcard beats a highlighter. Rereading feels productive because the text is familiar, and familiarity is not the same thing as being able to produce the answer when the page is shut.

Put the two together and you get the rule the whole idea runs on: review at the point where recall is still possible but no longer easy. Too early and you learn nothing, because you never forgot. Too late and you are learning it again from scratch.

What the intervals actually look like

A typical schedule for one item, assuming you recall it correctly each time:

The exact numbers vary between systems and they matter less than people assume. What matters is the shape: each gap is roughly double the last. Forget an item and it drops back down the ladder, because the gap you just tried was too long for that particular thing.

Why it usually fails in practice

The technique is sound and the software is the problem. Three failures account for most people giving up:

• The backlog. Miss a fortnight and you return to four hundred due cards. Most people never clear that and quietly stop.
• The setup. Decks, subdecks, note types, card templates. Time spent arranging a system is time not spent learning, and it is a very comfortable way to feel productive.
• Adding too much. Every item you add is a small recurring debt. A thousand items is a daily obligation, not a library.

None of these are problems with spacing. They are problems with being handed a scheduling system and asked to administer it.

What it is good for, and what it is not

It works on anything with a right answer you could say out loud: vocabulary, definitions, dates, formulas, drug doses, the names of the bones in the wrist, a line of poetry, someone's job title. It is excellent at making a fact available without effort, which is the foundation that thinking sits on.

It does not teach understanding. Reviewing the definition of a derivative on a schedule will not let you do calculus. The honest way to use it is to learn the thing properly once, by whatever means, and then use spacing to stop that understanding from decaying into a vague sense that you used to know it.

Also worth reading: how to remember what you read and how this compares to Anki.