The Spacing Effect: Why Timing Matters for Memory

What is the spacing effect?

When you study something more than once, the timing matters. Studying the same material in separate sessions with time in between, called spaced or distributed practice, tends to give better long-term memory than packing the same study into one uninterrupted block, called massed practice ^[1].

Two intervals matter. The gap is the time between study sessions. The retention interval is the time from the last session to the test ^[2]. Most of what follows is about how those two interact.

The effect is old news in psychology. Researchers began to demonstrate benefits of distributed practice in the late 1800s, among them Hermann Ebbinghaus, and the topic has been a mainstay of memory research ever since ^[1]. Ebbinghaus is better known for his forgetting curve, which we explain in Why do we forget things?

Spacing is not the same as interleaving. Interleaving means mixing different kinds of problems within a session; spacing is about the time between repetitions of the same material. The two often come together in practice; the interleaving method page explains how to practise interleaving.

What the meta-analyses found

Verbal learning. A 2006 meta-analysis found 839 assessments of distributed practice in 317 experiments, reported in 184 articles, all using verbal tasks such as word lists, word pairs, facts and texts ^[1]. Averaged across all retention intervals, people recalled 47% of the material after spaced study and 37% after massed study, and only 12 of 271 comparisons showed no benefit or a cost of spacing ^[1]. The gap that gave the best retention increased as the retention interval increased ^[1].

Practising tasks and skills. An earlier meta-analysis of 63 studies of task performance found an average effect of 0.46 in favour of spaced practice ^[7]. The size of the benefit depended on the kind of task and on the interval between practice trials, and studies with weaker methods reported larger effects than more rigorous ones ^[7].

Second languages. Across 48 experiments with 3,411 learners, spacing second-language practice had a medium-to-large effect, and longer gaps beat shorter ones on delayed tests ^[8].

Testing yourself, spaced. A 2021 meta-analysis of 29 studies found a strong benefit when retrieval practice, such as answering flashcards, was spaced out rather than massed (g = 0.74) ^[6].

A 2013 review of ten study techniques rated distributed practice as one of only two with high utility ^[9].

The gap depends on when you need to remember

A 2008 online study, which its authors suspected was the most systematic analysis of long-term spacing carried out up to then, tested timing in detail ^[2]. In it, 1,354 adults, with a mean age of 34 (range 18 to 72), learned 32 obscure trivia facts, such as which European nation consumes the most spicy Mexican food ^[2]. After a gap of anything from a few minutes to 105 days, they reviewed each fact twice by answering it and seeing the correct answer. They then took a final test 7, 35, 70 or 350 days later ^[2].

For every test delay, recall first rose as the gap grew and then slowly fell ^[2]. Compared with reviewing straight away, the best gap raised final recall by 64% overall, with the amount of study time held the same ^[2]. The best gap grew with the test delay, but shrank as a share of it ^[2]:

Two details matter for planning. First, a gap longer than the best one cost much less than a gap that was too short: accuracy rose steeply as the gap grew and then declined only gradually ^[2]. Second, there is no single best gap. As the authors put it, if you want to know the best way to distribute your study time, you need to decide how long you wish to remember something ^[2].

These figures come from one study of trivia facts with a single review session, so treat them as rough guides rather than a timetable.

Spacing versus massing in classrooms

Most spacing research has taken place in laboratories. A handful of studies have tested spacing directly in classrooms or with course-like material.

• Vocabulary. In fifth-grade classrooms, 39 children learned unfamiliar English words with both massed learning and learning spaced a week apart. Tested five weeks later, one-week spacing gave better retention than massed learning ^[3].
• History facts. Eighth-grade students reviewed US history facts one week or 16 weeks after learning them, or not at all. Tested nine months later, facts reviewed by answering questions with feedback were remembered better than restudied facts, and best of all when the review came after 16 weeks; the authors note that the gains were numerically small because overall scores were close to the floor ^[4].
• Mathematics. In two experiments with 216 college students, splitting ten practice problems across two sessions a week apart made no difference on a test one week later, but a very large difference on a test four weeks later ^[10]. Extra practice problems in a single session, known as overlearning, had no effect on later scores ^[10].

A 2016 policy review argued that spaced practice is a feasible and low-cost way to improve learning in schools, and discussed the barriers that keep it from being widely adopted ^[11]. The 2008 study's authors also pointed to a common practice that works against spacing, confining a topic to a single week of a course, which they warned is likely to produce high immediate mastery that does not last ^[2].

Why cramming feels better

If spacing works, why do people mass their study? Partly because massing feels effective.

In three flashcard experiments, people learned word pairs either in one large stack, which spaced the repetitions of each card, or in four small stacks studied one after another, which massed them ^[5]. Spacing was more effective than massing, and also more effective than cramming everything on the day before the test. It helped 90% of participants, yet after the first session 72% believed massing had been more effective ^[5].

In another set of experiments, people studied paintings by different artists, with each artist's paintings shown either together or mixed in with other artists' work ^[12]. Spreading out each artist's paintings made people better at recognising that artist's style in new paintings, but participants rated the massed version as more effective, even after their own test results had shown the opposite ^[12].

Timing of the check matters too. In the mathematics study, a test one week later showed no benefit of spacing at all ^[10]. If you judge a study method by how you do soon afterwards, massing can look as good as spacing.

Do the gaps need to grow?

Many study guides and apps space reviews at growing intervals, such as one day, then three, then a week. The evidence that growing gaps beat equal gaps is weak.

• In experiments with vocabulary word pairs, growing gaps between tests gave better recall 10 minutes later, but equal gaps gave better recall two days later. Delaying the first test, so that recall was harder, improved long-term retention whatever the spacing of the later tests ^[13].
• A meta-analysis of 54 effect sizes found no significant difference between growing and equal schedules of retrieval practice (g = 0.034). Growing gaps did better in studies where items were tested more often, and the authors concluded that their results do not support the widely held belief that intervals should be progressively increased ^[6].
• In second-language learning, equal and growing gaps were statistically equivalent ^[8].

The 2006 meta-analysis found only 22 relevant comparisons. Taken together they favoured growing gaps, but results varied so much between studies that the authors called this tentative and concluded that there was insufficient data to say whether growing gaps help; they also noted that this had not stopped some software developers from assuming that they do ^[1].

Why does spacing work?

There is no settled answer, and the theories below are background, not evidence for any particular schedule.

One family of theories, known as encoding variability, holds that spaced sessions take place in more varied mental contexts, and that this variety is what helps. In a 2010 review, Aaron Benjamin and Jonathan Tullis argued that these theories cannot explain some key findings, such as the fall in recall when gaps become very long, and proposed instead that a later study session works partly by reminding you of the earlier one ^[14]. Another account stresses difficulty: in the word-pair experiments above, what mattered most was delaying the first test so that recalling became more effortful ^[13].

How Recall Cards schedules reviews

Recall Cards uses FSRS-4.5, an open-source spaced-repetition scheduler, with its default settings. For each card it keeps an estimate of how stable your memory of it is and how difficult it is for you. After each review it sets the next one for when your predicted chance of recalling the card has fallen to about 90%.

For a new card, the four ratings lead to different gaps: Again brings the card back in the same sitting after a few other cards, Hard schedules it for the next day, Good for four days later and Easy for 14 days later. If you keep reviewing a card on time and rating it Good, the gaps grow: four days, then about 15, then about 50. Each session shows the cards that are due first, then up to ten new cards from each deck.

So Recall Cards uses growing gaps, adjusted to your ratings. The scheduler does not ask when your exam is: it aims to keep each card at about 90% predicted recall, not to time reviews for a particular test date.

We have not tested whether the FSRS schedule in Recall Cards gives better results than a simpler spacing schedule. Its 90% recall target is a design choice, not a research finding. For how to space reviews with paper cards or any other tool, see the spaced repetition method page; for a weekly plan that combines spacing with testing yourself, see How to improve memory for studying. Spreading study over several days also puts nights of sleep between sessions, which has its own research: see Sleep and memory.

What the research says

Frequently asked questions

How far apart should I space my study? It depends on when you need the material. In the 2008 study, the best gap was a few days for a test a week away and about a month for a test nearly a year away ^[2]. If in doubt, err on the long side: gaps that were too long cost much less than gaps that were too short ^[2].

Is cramming useless? It may do as well on a test soon afterwards, but what you cram tends not to last. In the mathematics study, massed practice did as well as spaced practice on a test a week later and much worse four weeks later ^[10], and in flashcard experiments spacing beat cramming on the day before the test ^[5].

Does spacing work for skills, not just facts? On average, yes: across 63 studies of task performance, spaced practice gave better results, but the size of the benefit depended on the task ^[7]. Spacing maths practice also helped on a delayed test ^[10].

References

1. 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
2. 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
3. Sobel, H. S., Cepeda, N. J., & Kapler, I. V. (2011). Spacing effects in real‐world classroom vocabulary learning. Applied Cognitive Psychology, 25(5), 763–767. https://doi.org/10.1002/acp.1747
4. Carpenter, S. K., Pashler, H., & Cepeda, N. J. (2009). Using tests to enhance 8th grade students' retention of U.S. history facts. Applied Cognitive Psychology, 23(6), 760–771. https://doi.org/10.1002/acp.1507
5. Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297–1317. https://doi.org/10.1002/acp.1537
6. 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
7. 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
8. Kim, S. K., & Webb, S. (2022). The effects of spaced practice on second language learning: A meta-analysis. Language Learning, 72(1), 269–319. https://doi.org/10.1111/lang.12479
9. 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. https://doi.org/10.1177/1529100612453266
10. Rohrer, D., & Taylor, K. (2006). The effects of overlearning and distributed practise on the retention of mathematics knowledge. Applied Cognitive Psychology, 20(9), 1209–1224. https://doi.org/10.1002/acp.1266
11. Kang, S. H. K. (2016). Spaced Repetition Promotes Efficient and Effective Learning: Policy Implications for Instruction. Policy Insights from the Behavioral and Brain Sciences, 3(1), 12–19. https://doi.org/10.1177/2372732215624708
12. Kornell, N., & Bjork, R. A. (2008). Learning Concepts and Categories: Is Spacing the “Enemy of Induction”? Psychological Science, 19(6), 585–592. https://doi.org/10.1111/j.1467-9280.2008.02127.x
13. 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. https://doi.org/10.1037/0278-7393.33.4.704
14. Benjamin, A. S., & Tullis, J. (2010). What makes distributed practice effective? Cognitive Psychology, 61(3), 228–247. https://doi.org/10.1016/j.cogpsych.2010.05.004