Spaced repetition: what the research actually says

The short answer

Spacing works. When the same amount of study is spread out over time rather than packed into one session, people remember more, for longer. In their 2013 review of ten common study techniques, Dunlosky and colleagues rated distributed practice as one of only two techniques with high utility, alongside practice testing [1].

The benefit is not small, and it grows with the delay. What makes spacing hard to use is that it does not feel better while you are doing it.

What the studies found

Spacing beats massing, even for the same cards. In Kornell's 2009 flashcard experiments, students studied vocabulary either in one big spaced stack or in several small massed ones. Spacing produced better recall for 90% of participants (63 of 70), and in one experiment it doubled recall, 54% against 21% for massed study [2]. Each experiment was small (20 to 25 people), but the pattern held across all of them.

The longer you need to remember, the longer the gap should be. Cepeda and colleagues tested 1,354 people on 32 facts, with gaps between two study sessions of up to 105 days and final tests up to 350 days later [3]. Compared with no gap, the best gap improved recall by 64% (a relative increase, not 64 percentage points). The best gap got longer as the test got further away: roughly 3 days for a test a week later, 8 days for 5 weeks, 12 days for 10 weeks, and 27 days for a year. As a share of the time until the test, though, the best gap fell, from about 43% to about 8%. So "review at 10–20% of the time to the exam" is a rough guide at best, not a rule.

Longer gaps can keep more with fewer sessions. Over nine years, Bahrick and colleagues relearned foreign vocabulary on different schedules [4]. Thirteen sessions 56 days apart kept as much (57%) as twenty-six sessions 14 days apart (56%). There were only four participants, the authors themselves, so treat it as a striking illustration rather than proof.

It works in real courses. In a semester-long course, personalized spaced review gave 16.5% better retention a month after the course ended than massed review, and 10% better than a generic spacing schedule [5]. In a randomized trial with 724 urology residents, questions sent by email at spaced intervals produced larger long-term gains than the same questions bunched together (15.2% against 3.4%), though the two groups also differed in timing [6].

Why spacing feels wrong

In Kornell's study, 72% of the students who picked a side believed massed study had worked better, the opposite of their own results [2]. After a massed session the material feels fresh and fluent, and that feeling is easy to mistake for learning. Spaced review feels harder because you have partly forgotten, and that effort is part of why it works. Researchers call this a desirable difficulty: performance during practice "is often an unreliable index" of how much you are learning [7].

Do expanding intervals matter?

Many apps grow the gap after each successful review: one day, then three, then a week, and so on. A meta-analysis by Latimier, Peyre and Ramus found that spaced retrieval clearly beats massed retrieval (g = 0.74), but expanding gaps were no better than equal ones (g = 0.03) [8]. What seems to matter more is that the first review comes after a real delay rather than straight away [9], and that each review is a genuine attempt to recall. More correct recalls inside one session give diminishing returns; a longer gap between successful recalls gives more [10].

Repetition has a ceiling: overlearning

Doing more practice in the same session does not buy much. In Rohrer and Taylor's maths study, spreading four problems across two sessions scored 74% a week later against 49% for doing all four at once, and doubling the massed practice brought no extra benefit [11]. The return comes from coming back, not from staying longer.

How to use spacing with lectures

• Review each lecture again a day or more later, after you have slept on it, not the same evening.
• Keep reviewing what you already "know." Students tend to drop a card after one correct answer; that habit cut recall roughly in half in one experiment [12]. A useful target from the research on successive relearning: get it right about three times, then relearn it in about three spaced sessions [13].
• Let the schedule follow the exam. For a final in ten weeks, gaps of a week or two are reasonable; for something you need next year, longer.
• Spread your reviews, not your hours. Three short sessions on different days beat one long one.
• Use a scheduler. Keeping track of when every item is due is exactly the bookkeeping software does well. Modern open schedulers such as FSRS fit their predictions to millions of real reviews [14].

Where the evidence stops

Most spacing experiments use facts, words or short passages, with two or three sessions. Fewer test complex reasoning or a full course over months, and the best gap depends on the material and on how well it was learned in the first place. Spacing is also not a substitute for understanding: it keeps what you have learned, so it pays to learn it well first. See retrieval practice for the other half of the picture.

References

1. Dunlosky, J., Rawson, K. A., Marsh, E. J., Nathan, M. J., & Willingham, D. T. (2013). Improving students’ learning with effective learning techniques. Psychological Science in the Public Interest, 14(1), 4–58. doi.org/10.1177/1529100612453266
2. Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297–1317. doi.org/10.1002/acp.1537
3. Cepeda, 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), 1095–1102. doi.org/10.1111/j.1467-9280.2008.02209.x
4. Bahrick, 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), 316–321. doi.org/10.1111/j.1467-9280.1993.tb00571.x
5. Lindsey, R. V., Shroyer, J. D., Pashler, H., & Mozer, M. C. (2014). Improving students’ long-term knowledge retention through personalized review. Psychological Science, 25(3), 639–647. home.cs.colorado.edu/~mozer/Research/Selected%20Publications/reprints/LindseyShroyerPashlerMozer2014Published.pdf
6. Kerfoot, B. P., Fu, Y., Baker, H., Connelly, D., Ritchey, M. L., & Genega, E. M. (2010). Online spaced education generates transfer and improves long-term retention of diagnostic skills: A randomized controlled trial. Journal of the American College of Surgeons, 211(3), 331–337.e1. (Abstract.) doi.org/10.1016/j.jamcollsurg.2010.04.023
7. Soderstrom, N. C., & Bjork, R. A. (2015). Learning versus performance: An integrative review. Perspectives on Psychological Science, 10(2), 176–199. doi.org/10.1177/1745691615569000
8. Latimier, A., Peyre, H., & Ramus, F. (2021). A meta-analytic review of the benefit of spacing out retrieval practice episodes on retention. Educational Psychology Review, 33(3), 959–987. (Abstract.) doi.org/10.1007/s10648-020-09572-8
9. Karpicke, J. D., & Roediger, H. L. (2007). Expanding retrieval practice promotes short-term retention, but equally spaced retrieval enhances long-term retention. Journal of Experimental Psychology: Learning, Memory, and Cognition, 33(4), 704–719. profiles.wustl.edu/en/publications/expanding-retrieval-practice-promotes-short-term-retention-but-eq/
10. Pyc, M. A., & Rawson, K. A. (2009). Testing the retrieval effort hypothesis: Does greater difficulty correctly recalling information lead to higher levels of memory? Journal of Memory and Language, 60(4), 437–447. doi.org/10.1016/j.jml.2009.01.004
11. Rohrer, D., & Taylor, K. (2007). The shuffling of mathematics problems improves learning. Instructional Science, 35(6), 481–498. doi.org/10.1007/s11251-007-9015-8
12. Kornell, N., & Bjork, R. A. (2008). Optimising self-regulated study: The benefits—and costs—of dropping flashcards. Memory, 16(2), 125–136. doi.org/10.1080/09658210701763899
13. Rawson, K. A., & Dunlosky, J. (2011). Optimizing schedules of retrieval practice for durable and efficient learning: How much is enough? Journal of Experimental Psychology: General, 140(3), 283–302. doi.org/10.1037/a0023956
14. Open Spaced Repetition. SRS benchmark: FSRS and other algorithms evaluated on 349.9 million Anki reviews. github.com/open-spaced-repetition/srs-benchmark