What is spaced repetition? Why spreading study out works

Spaced repetition is a way of studying in which you review the same material several times with growing gaps between reviews, instead of all at once. Each review comes as the memory is starting to fade, which makes it stronger and slower to fade next time. It rests on the spacing effect, one of the most reliable findings in the psychology of memory.

In short

• Memory fades fast at first and then more slowly; a review just as it starts to fade resets the curve and flattens it.
• Spreading the same study time over days beats cramming it into one session, often by a wide margin, for anything you need to remember for more than a few days.
• It works best combined with retrieval practice: testing yourself, not rereading.

Where does the idea come from?

Between 1879 and 1885, the German psychologist Hermann Ebbinghaus ran the first careful experiments on memory, using himself as the subject. He memorized lists of nonsense syllables, such as "DAX" or "BOK," chosen so that meaning wouldn't help him, and then measured how much time he saved when he relearned a list after a delay. He published the results in 1885 in Memory (Ebbinghaus, 1885).

His savings fell steeply at first and then leveled off:

Time after learning	Share of the original effort saved on relearning
20 minutes	58%
1 hour	44%
About 9 hours	36%
1 day	34%
2 days	28%
6 days	25%
31 days	21%

That shape is the forgetting curve. In 2015, researchers repeated his experiment with a new subject and found a curve that closely resembled the original (Murre and Dros, 2015). Ebbinghaus also found that spreading his repetitions over several days needed fewer repetitions in all than massing them into one day, an early demonstration of the spacing effect.

Why does spacing work?

The short answer is that a memory gets stronger when you have to work a little to bring it back. If you review something you saw a minute ago, it is still fresh, so the review costs little effort and adds little. If you wait until it has partly faded, recalling it takes effort, and that effort strengthens the memory and slows its next decline. Each successful review lets the next gap be longer.

Psychologists have several overlapping explanations. Spaced reviews happen in different contexts and moods, which gives the memory more cues to be found by later. Retrieving something after a gap rebuilds it more thoroughly than seeing it again while it's fresh. And massed study gives a feeling of fluency (it seems easy, so it seems learned), which is misleading. What fades fastest is the material that felt easiest to cram.

How long should the gaps be?

That depends on how long you need to remember. In one of the largest studies, Nicholas Cepeda and colleagues had 1,354 people learn a set of facts, review them once after a gap of up to 3.5 months, and take a final test up to a year after that review (Cepeda and colleagues, 2008). For a fixed amount of study time, the best gap improved final recall by 64% compared with reviewing immediately. The best gap grew as the test got further away, but more slowly than the delay itself:

Final test after	Best gap between sessions (recall)	Gap as a share of the delay
7 days	about 3 days	43%
35 days	about 8 days	23%
70 days	about 12 days	17%
350 days	about 27 days	8%

The practical lesson is that the gap should be a sizable fraction of the time you need to remember, and that a gap that is too long costs less than one that is too short.

How do you schedule reviews?

You don't need software. A paper method works, and the principle is simple: review at expanding intervals, and if you get something wrong, start its intervals again.

A worked schedule. Say you have an exam in 30 days and a set of 40 vocabulary words. Learn them on day 1, then review on days 2, 4, 8, 15 and 25, each gap longer than the last, finishing a few days before the exam. At each review, test yourself before you look. Words you miss go back to a one-day gap; words you get right move on. That is 6 sessions in all instead of one long night, and by the last review most words take seconds.

The Leitner box. In his 1972 book So lernt man lernen (How to learn), the German science writer Sebastian Leitner described a way to do this with flashcards and a box with several compartments. Every card starts in box 1. Get a card right and it moves up a box; get it wrong and it goes back to box 1. Each box is reviewed less often than the one before; one common spacing is:

Box	Review every	A card is here because
1	Day	It's new, or you just got it wrong
2	2 days	You got it right once
3	4 days	Right twice in a row
4	8 days	Right three times
5	16 days	Well learned

Flashcard apps do the same thing automatically, adjusting each card's next review to how easily you recalled it. The scheduling matters more than the tool.

Why test yourself instead of rereading?

Spacing tells you when to review; retrieval practice tells you how. Rereading notes feels productive but is mostly recognition: the material looks familiar, so it seems known. Closing the notes and trying to recall the answer, from a flashcard, a practice question or a blank page, is harder, and that difficulty is what makes it work. A 2013 review of ten common study techniques rated practice testing and distributed practice the only two of "high utility," and found highlighting and rereading of low utility (Association for Psychological Science).

A good flashcard asks one thing, in your own words, with a clear answer. For ideas rather than facts, the better test is to explain the idea from memory, which is the heart of the Feynman technique: explain it simply, notice where you get stuck, and go back to the source for that part.

Still argued

That spacing beats cramming is not in serious dispute; it has been replicated across hundreds of studies, ages and subjects. What researchers still argue about is the details: whether intervals should expand or stay equal, how to set them for complex skills rather than facts, and how much the benefit carries over to the understanding tested in real courses. Most studies use word lists, facts or vocabulary, and few test memory over long stretches: of more than 400 reports in one 2006 review, only about a dozen tested recall a day or more later (Cepeda and colleagues, 2008).

Spaced repetition fits neatly with short, focused study sessions like the Pomodoro Technique, and it is how people who learn across many fields, like polymaths, keep earlier subjects from fading while they move on. For what is happening in the brain when a memory strengthens, see how the brain works. And questions about what it means to know something, beyond being able to recall it, belong to philosophy.

Check yourself

• Why does a review just as you start to forget strengthen a memory more than one straight away?
• For a test a month away, roughly how long should the gap between study sessions be, and why not just one day?
• How does a Leitner box decide when you see a card again?

Sources

• Memory: A Contribution to Experimental Psychology — Hermann Ebbinghaus, 1885, translated 1913, via Classics in the History of Psychology
• Replication and Analysis of Ebbinghaus' Forgetting Curve — Jaap M. J. Murre and Joeri Dros, PLOS ONE, 2015
• Spacing Effects in Learning: A Temporal Ridgeline of Optimal Retention — Cepeda, Vul, Rohrer, Wixted and Pashler, Psychological Science, 2008
• Improving Students' Learning With Effective Learning Techniques — John Dunlosky and colleagues, Association for Psychological Science, 2013