Spaced repetition: why reviewing later beats reviewing more

You read the chapter three times on Sunday night. On Monday morning it all felt familiar. Two weeks later, in the actual exam, half of it was gone. That is not a sign that your memory is bad. It is how memory works for everyone, and it is one of the oldest findings in psychology.

The short version: you forget fast at first, then more slowly. If you review just as something starts to fade, and leave a bigger gap each time, the same amount of study time lasts far longer than cramming it all in one sitting.

What spaced repetition is

Spaced repetition means you review the same material several times, with gaps between the sessions, and the gaps grow as the memory gets stronger. Today, then in a day or two, then in a week, then in a few weeks.

The opposite is massed practice, which most of us know as cramming: all the repetitions packed into one session. Cramming feels productive because the material is fresh in your head while you study. Spacing feels harder because you have partly forgotten it each time you come back. That difficulty is the point, as you will see below.

The key studies

It started with one stubborn man and a pile of nonsense syllables. In the early 1880s, the German psychologist Hermann Ebbinghaus memorized lists of made-up syllables (like "DAX" or "BOK") so that meaning could not help him. He learned each list until he could recite it perfectly, waited, then timed how long it took to relearn it. The time he saved told him how much was still in memory. After about an hour, he needed roughly half the original effort to relearn a list. After a day, about a third of the work was still saved. After a month, about a fifth. Memory drops steeply at first, then levels off. That shape is the forgetting curve (Ebbinghaus, 1885).

One person testing himself in the 1880s is a thin base, so in 2015 two researchers in Amsterdam ran it again. One subject spent 70 hours learning and relearning lists at gaps from 20 minutes to 31 days. The curve came out very close to the original (Murre & Dros, 2015).

The forgetting curve tells you why reviewing helps. The spacing effect tells you when. In 2006, Nicholas Cepeda and colleagues pulled together 317 experiments from 184 articles. Spaced study beat massed study on the final test across the board. They also found that the best gap between sessions gets longer the longer you need to remember (Cepeda et al., 2006).

Two years later, the same group ran one of the largest spacing studies ever. More than 1,350 people learned a set of obscure trivia facts, reviewed them after a gap of anywhere from a few minutes to 105 days, then took a final test up to a year later. For people tested 10 weeks after review, the best gap more than doubled how much they could recall compared with reviewing straight away (a 111 percent gain). The best gap also depended on the test date. For a test a few weeks to a couple of months away, it was roughly 10 to 20 percent of the time until the test. For a test a year away, it was closer to a month, or about 8 percent (Cepeda et al., 2008).

Does it hold over years? Harry Bahrick and three members of his family learned 300 foreign words each and tracked themselves for nine years. They reviewed every 14, 28 or 56 days, then were tested up to five years after training ended. Thirteen sessions spaced 56 days apart gave about the same long-term memory as 26 sessions spaced 14 days apart. Half the sessions, same result, just by spreading them out (Bahrick et al., 1993).

And with flashcards, the tool you actually use? Nate Kornell had students study vocabulary either in one big spaced deck or in small crammed stacks. Spacing worked better for 90 percent of them. Yet after the first session, 72 percent believed cramming had worked better (Kornell, 2009). Your feeling of "I know this" during study is a poor guide to what you will remember later.

When a team of five learning scientists rated ten popular study techniques against the evidence, only two got their top rating of "high utility": practice testing and distributed practice, which is spacing (Dunlosky et al., 2013).

Why it works

Nobody has a full answer, but two ideas explain most of it.

First, coming back after a gap makes you work to pull the memory up again, and that effort strengthens it. When you reread something five minutes later, it is still sitting in your head, so your brain does very little. When you come back after three days, you have to dig for it.

Second, each spaced review tells your brain this information keeps coming back, so it is worth keeping. Memories that get used at growing intervals last longer, and each review can wait a little longer than the last.

Where it falls short

Cramming is not useless. If your exam is tomorrow morning, a late session will help you tomorrow morning. The problem is next month: the final, the next course that builds on this one, the job where you need it. Crammed material fades fast. The Cepeda 2008 data show this too: for a test only a week out, the best gap beat no gap by just 10 percent. For a test 10 weeks out, it beat it by 111 percent.

Spacing also needs something to space. It helps you keep what you have learned, but it will not make a confusing topic clear. Understand it first, then space it.

The size of the boost depends on the material and the test. Most studies use facts, word pairs and vocabulary. The effect shows up for concepts and problem solving too, but those studies are fewer and the gains vary more.

There is no single magic schedule. "Review on day 1, 3, 7, 21" is a reasonable rule of thumb, not a law. What matters is that gaps grow and that you actually come back.

A common mistake: spacing out rereading. Spaced rereading beats crammed rereading, but spacing works best when each review makes you recall the answer from memory before you look. That is active recall, and the two work best together.

How to use it yourself

You can do this with paper, a notes app, or any flashcard tool.

1. 1Turn what you study into questions or prompts. One idea per prompt.
2. 2Do your first review the next day, not the same evening.
3. 3Each time you recall an idea correctly, push the next review further out: a few days, then a week or two, then a month or more.
4. 4Each time you miss one, bring it back soon and start the gaps again from short.
5. 5Mix in old material. Every daily session should include ideas from previous weeks, not only this week's lecture.
6. 6Work back from your exam date. For an exam in two months, a review gap of about a week or two around the middle is a good target.

Tip: keep sessions short and daily. Fifteen minutes every day beats two hours every Sunday, and a missed day is fine. Just pick up where you left off.

How lezrn uses it

When you finish a lesson in lezrn, each of its ideas goes into your review plan automatically. The schedule is set by FSRS, an open-source spaced repetition scheduler tested against hundreds of millions of real flashcard reviews. It aims for about 90 percent recall, so the gaps grow as you keep getting an idea right. The full details are on how review works and the spaced repetition page.

• An idea you keep getting right comes back roughly 1 day, 4 days, 2 weeks, then 7 weeks later.
• Each review shows the idea's name. You try to recall it, then see the definition and choose Forgot or Got it.
• Your daily review pulls due ideas from all your lessons, oldest due first. Missed reviews roll into today instead of piling up.
• You can set an exam date. Blurting and Feynman mode grades feed the same schedule (recall tools).

Sources

1. 1Ebbinghaus, H. (1885). Über das Gedächtnis. Leipzig: Duncker & Humblot. English translation by H. A. Ruger & C. E. Bussenius (1913), Memory: A contribution to experimental psychology. New York: Teachers College, Columbia University. https://psychclassics.yorku.ca/Ebbinghaus/index.htm
2. 2Murre, J. M. J., & Dros, J. (2015). Replication and analysis of Ebbinghaus' forgetting curve. PLOS ONE, 10(7), e0120644. https://doi.org/10.1371/journal.pone.0120644
3. 3Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354 to 380. https://doi.org/10.1037/0033-2909.132.3.354
4. 4Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095 to 1102. https://doi.org/10.1111/j.1467-9280.2008.02209.x
5. 5Bahrick, H. P., Bahrick, L. E., Bahrick, A. S., & Bahrick, P. E. (1993). Maintenance of foreign language vocabulary and the spacing effect. Psychological Science, 4(5), 316 to 321. https://doi.org/10.1111/j.1467-9280.1993.tb00571.x
6. 6Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297 to 1317. https://doi.org/10.1002/acp.1537
7. 7Dunlosky, J., Rawson, K. A., Marsh, E. J., Nathan, M. J., & Willingham, D. T. (2013). Improving students' learning with effective learning techniques: Promising directions from cognitive and educational psychology. Psychological Science in the Public Interest, 14(1), 4 to 58. https://doi.org/10.1177/1529100612453266