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Does spaced repetition work?

Yes: spreading reviews over time, instead of cramming, improves long-term memory, as hundreds of experiments since 1885 show. The evidence is strongest for facts and vocabulary in controlled studies; real-world studies are fewer, and the benefit is smaller for complex skills.

Updated 6 min read

In short

  • Spaced repetition works: spreading the same study time over days or weeks makes it stick better than one long session.
  • The evidence is strongest for facts and vocabulary, and spacing works even better when each review tests you, as flashcards do.
  • The evidence is large: hundreds of experiments since 1885, and major reviews rank spacing among the most useful study techniques.
  • It is weaker for complex skills and real classrooms, and studies of apps like Anki show a link with exam scores, not proof that the app caused it.

How strong is the evidence?

Strong: researchers have tested it for more than a century, and the major reviews agree that spacing improves long-term memory. Psychologists call this the spacing effect: the same amount of study is remembered better when it is spread out than when it is crammed into one session. Spaced repetition is the study method built on it, usually with flashcards.

Ebbinghaus, the German psychologist who first measured the forgetting curve, tested it on himself in 1885. Thirty-eight repetitions spread over the three days before a test worked as well as 68 made the day before.[1] By 2006, a meta-analysis (a study that pools the results of earlier studies) found hundreds of experiments comparing spaced and crammed study on verbal memory alone.[2]

  • A 1999 meta-analysis of 63 studies found that spaced practice beat practice in one block, by a margin close to what researchers call a medium effect.[3]
  • A 2013 review of 10 common study techniques gave its top rating to only two: spacing out practice and testing yourself. Rereading and highlighting were rated low.[4]
  • A 2016 review reached the same conclusion: spaced practice gives better long-term learning than cramming.[5]

How far apart should reviews be?

The longer you need to remember something, the longer the gaps between reviews should be.[2] In a 2008 study, people learned facts, reviewed them after a gap, and took a final test up to a year later. For a test a year away, the best gap before the review was roughly 18 to 36 days; for a test a week away, the best gap was a larger share of the wait.[6]

Many apps lengthen the gap after each successful review. A 2021 meta-analysis confirmed that spacing out self-tests beats bunching them, but found that growing gaps were not reliably better than equal ones.[7] Software cannot know the moment you will forget a card. Algorithms such as FSRS instead estimate the chance you can still recall it, and schedule the next review when that chance falls to a target, such as 90%. For practical schedules, see spaced repetition schedules.

Does spaced repetition work outside the lab?

Yes, over years and in real teaching, but these studies are fewer and smaller than lab experiments, and one large trial found no effect.

  • Over years. In a 1993 study, Bahrick and three other psychologists learned foreign words themselves and were tested up to five years later. Reviewing every 56 days, they needed half as many sessions as reviewing every 14 days for similar results.[8] With four people, it shows what is possible, not what to expect.
  • In a classroom setting. After a simulated university lecture, students who reviewed 8 days later did better on a later test than those who reviewed after 1 day, on both factual and higher-level questions.[9]
  • In medical training. Short clinical questions sent by email at intervals, which Kerfoot and colleagues called “spaced education”, raised medical students' end-of-year scores on the topics covered in a 2007 trial,[10] and helped residents more than the same questions given as web modules in a 2010 trial.[11]
  • A trial with no effect. In 2021, app alerts encouraging spaced practice questions did not get Canadian family medicine residents to answer more of them, and exam scores did not differ.[12] Spacing only helps if people actually do the reviews.

Does using Anki improve exam results?

Anki use is linked to higher scores on the USMLE Step 1, the first US medical licensing exam, but the studies show a link, not that Anki caused it. Students who choose Anki may differ from others in ways the studies cannot measure.

In a 2015 survey at one US medical school, the more unique Anki cards students had seen, the higher their Step 1 score, even after accounting for entrance-exam scores, grades and test anxiety.[13] A 2026 review of 11 studies found that frequent users scored 4 to 13 points higher on Step 1 than minimal users in three of them. Results for course exams were mixed, and the one study of Step 2 CK, the second licensing exam, found no clear benefit.[14]

Should you test yourself when you review?

Yes: spacing works better when each review is a test, which is how flashcard apps already work.[5] Testing yourself is called retrieval practice, and it is the other technique the 2013 review rated highest.[4]

In a 2008 experiment, once students had recalled a foreign word correctly, studying it again did nothing for a later test, but testing it again helped a lot.[15] Spacing those tests out also beats doing them all at once.[7] For how the two ideas fit together, see active recall.

Where is the evidence weaker?

The evidence is weaker for complex skills, for real classrooms, and for long-term app use outside research.

  • Complex skills. The more complex the task, the smaller the benefit of spacing.[3] Some studies find benefits for higher-level questions and problem solving,[5][9] but far fewer than for facts and word lists.
  • Real classrooms. Most experiments use short delays in the lab, and relatively few have tested spacing in real teaching.[6][9]
  • Sticking with it. Longer gaps can make learning slightly slower at first,[8] and reminders alone did not get residents to practise more.[12]
  • App studies. Studies of Anki are mostly observational and measure “use” in different ways. Mixed results may reflect deck quality, how regularly students used the app, and how long they had to prepare.[14] Spacing cannot fix unclear cards.

Which studies is this page based on?

The main studies and reviews cited on this page, in order of publication:

StudyWho or whatMain finding
Ebbinghaus (1885)Himself, nonsense syllables38 spread repetitions matched 68 in one day[1]
Bahrick et al. (1993)4 psychologists, 9 years13 sessions 56 days apart matched 26 sessions 14 days apart[8]
Donovan & Radosevich (1999)Meta-analysis, 63 studiesSpacing better (d = 0.46), less so for complex tasks[3]
Cepeda et al. (2006)Meta-analysis, 317 experimentsLonger test delay, longer best gap[2]
Kerfoot et al. (2007)95 medical students, randomisedEmailed questions raised year-end scores[10]
Cepeda et al. (2008)1,350+ people, up to 1 yearBest gap: 20–40% of a 1-week delay, 5–10% of a 1-year delay[6]
Kerfoot et al. (2010)724 residents, randomisedScores up 15.2% with spaced emails, 3.4% with web modules[11]
Dunlosky et al. (2013)Review of 10 techniquesSpacing and self-testing rated highest[4]
Kapler et al. (2015)169 students, simulated lecture8-day review beat 1-day review[9]
Deng et al. (2015)72 medical students, surveyAnki cards seen predicted Step 1 (correlational)[13]
Latimier et al. (2021)Meta-analysis, spaced self-testingSpacing helps; expanding gaps no better than equal ones[7]
Grad et al. (2021)634 residents, cluster randomisedAlerts: no more practice, no score difference[12]
Frappa et al. (2026)Review of 11 Anki studiesHigher Step 1, mixed course exams; mostly observational[14]
d measures the gap between two groups in standard deviations; 0.5 is conventionally called a medium effect. Percentages are as reported by the authors.

Sources

  1. Ebbinghaus, H. (1913). Memory: A contribution to experimental psychology (H. A. Ruger & C. E. Bussenius, Trans.), section 34. New York: Teachers College, Columbia University. (Original work published 1885.) https://psychclassics.yorku.ca/Ebbinghaus/memory8.htm
  2. Cepeda, 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), 354–380. https://doi.org/10.1037/0033-2909.132.3.354
  3. Donovan, J. J., & Radosevich, D. J. (1999). A meta-analytic review of the distribution of practice effect: Now you see it, now you don't. Journal of Applied Psychology, 84(5), 795–805. https://doi.org/10.1037/0021-9010.84.5.795
  4. Dunlosky, 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), 4–58. https://doi.org/10.1177/1529100612453266
  5. Kang, S. H. K. (2016). Spaced repetition promotes efficient and effective learning. Policy Insights from the Behavioral and Brain Sciences, 3(1), 12–19. https://doi.org/10.1177/2372732215624708
  6. 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. https://doi.org/10.1111/j.1467-9280.2008.02209.x
  7. 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. https://doi.org/10.1007/s10648-020-09572-8
  8. 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. https://doi.org/10.1111/j.1467-9280.1993.tb00571.x
  9. Kapler, I. V., Weston, T., & Wiseheart, M. (2015). Spacing in a simulated undergraduate classroom: Long-term benefits for factual and higher-level learning. Learning and Instruction, 36, 38–45. https://doi.org/10.1016/j.learninstruc.2014.11.001
  10. Kerfoot, B. P., DeWolf, W. C., Masser, B. A., Church, P. A., & Federman, D. D. (2007). Spaced education improves the retention of clinical knowledge by medical students: A randomised controlled trial. Medical Education, 41(1), 23–31. https://doi.org/10.1111/j.1365-2929.2006.02644.x
  11. 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. https://doi.org/10.1016/j.jamcollsurg.2010.04.023
  12. Grad, R., Leger, D., Kaczorowski, J., Schuster, T., Adler, S., et al. (2021). Does spaced education improve clinical knowledge among Family Medicine residents? A cluster randomized controlled trial. Advances in Health Sciences Education, 26(3), 771–783. https://doi.org/10.1007/s10459-020-10020-z
  13. Deng, F., Gluckstein, J. A., & Larsen, D. P. (2015). Student-directed retrieval practice is a predictor of medical licensing examination performance. Perspectives on Medical Education, 4(6), 308–313. https://doi.org/10.1007/s40037-015-0220-x
  14. Frappa, N. M., Chernov, D. E., Dillon, M. R., & Alben, M. G. (2026). Anki use and academic performance in medical education: A systematic review of evidence and learning theory. Medical Science Educator, 36(2), 1015–1025. https://doi.org/10.1007/s40670-026-02643-5
  15. Karpicke, J. D., & Roediger, H. L. (2008). The critical importance of retrieval for learning. Science, 319(5865), 966–968. https://doi.org/10.1126/science.1152408