Spaced Repetition: How to Schedule Reviews So You Remember

What is spaced repetition?

Spaced repetition is a way of scheduling review. Instead of studying a topic heavily for one evening and leaving it, you return to it several times, with a longer gap before each return. The first review might come a day after you learn something, the next a few days later, then a week, then a month. Material you know well drifts toward long intervals and costs almost nothing to keep; material you keep forgetting stays on short intervals until it holds.

The method rests on two of the most reliable findings in memory research. The first is the forgetting curve, described by the German psychologist Hermann Ebbinghaus in the 1880s after years of testing his own memory for lists of nonsense syllables: forgetting is fast at first, then slows. The second is the spacing effect: for the same total study time, reviews spread over days produce longer-lasting memory than reviews bunched together.

Why spaced repetition beats cramming

Cramming does work, briefly. Hours of review the night before can get you through a test the next morning, because the material is still fresh. What it does not do is last. A week later much of it has faded, which becomes a real problem in any course where later topics build on earlier ones, and in final exams that cover the whole year.

Spacing works because of the forgetting in between. When you return to material you have partly forgotten, retrieving it takes effort, and that effortful retrieval strengthens the memory more than an easy review of something you saw ten minutes ago. That is why each review should be a recall attempt, not a reread. Spacing tells you when to review; active recall tells you how. Together they are among the best-supported habits in research on how students learn.

A spaced repetition schedule you can start with

There is no single correct set of intervals. Research suggests the best gap depends on how long you need to remember something, and flashcard software sets intervals for each item individually. If you are scheduling by hand, this pattern is a sensible default for a topic you learned today.

Two adjustments make the schedule respond to you. If a review goes badly, for instance you miss more than a couple of key points, bring the next review forward to a day or two. If it goes easily, stretch the next gap. And if your exam is close, compress the whole pattern: the same expanding shape still helps when it fits into ten days instead of two months.

The Leitner system: spaced repetition with a box of cards

If you prefer paper, the Leitner system turns spaced repetition into a set of boxes. It was devised by the German science writer Sebastian Leitner in the 1970s and needs nothing more than index cards and a few dividers. Cards move forward a box when you answer them correctly and go back to the first box when you miss them, so your hardest cards come up most often without any calculation on your part.

1. 1Label five boxes or sections. One common pattern: Box 1 reviewed every day, Box 2 every two days, Box 3 twice a week, Box 4 once a week, Box 5 every two weeks.
2. 2Put every new card in Box 1.
3. 3Each study day, review the boxes that are due, answering every card aloud or on paper before you turn it over.
4. 4Move each card you answer correctly up one box.
5. 5Send every card you miss back to Box 1, however far along it was.
6. 6Set aside cards that stay correct in Box 5 for several rounds, and bring them back out for one last check before the exam.

How spaced repetition apps schedule your reviews

Flashcard apps with spaced repetition do the box-moving for you. After each card, you rate how well you recalled it, usually on a scale running from “again” to “easy.” The app stretches the card's interval each time you get it right, so a card might return after a day, then a few days, then weeks, then months, and it drops the card back to a short interval when you get it wrong. Each card ends up on its own schedule. The cards themselves still need care, which the guide to making flashcards covers.

The trap is review debt. Every new card creates reviews for weeks to come, so adding a large batch each day quietly builds a daily queue you cannot finish. Once the queue gets long, people skip a few days, the backlog grows, and they abandon the system. Pick a number of new cards per day that leaves you a comfortable daily review time, and treat finishing the day's reviews as more important than adding anything new.

Spacing more than flashcards: problems, essays and whole topics

Spaced repetition is often treated as a flashcard technique, but the principle applies to anything you can practice. Plenty of material fits badly on a card: a proof, a lab method, the argument of an essay. You can still space it.

Mixing old topics into new sessions has a second benefit. It forces you to work out which idea or method a question needs, which is exactly what exams test. That practice is called interleaving, and the guide to studying math shows it at work on mixed problem sets. Some ways to space material that does not suit cards:

• Problem sets: start each session by re-solving two or three problems from a topic covered weeks ago, without looking at your old solutions.
• Essay subjects: once a week, outline from memory an answer to an old essay question, then check it for missing evidence.
• Law and history: keep a running list of cases or events and quiz yourself on why each one matters, not just its name.
• Languages: reuse old vocabulary in new sentences you write or say aloud, instead of only flipping cards.
• Whole courses: reserve a short weekly slot for a cumulative self-test that reaches back to the first week of term.