The spacing effect

What is the spacing effect?

The spacing effect is the finding that study spread over several sessions leads to better long-term memory than the same amount of study massed into one.^[1] Massed study means repeating something back to back, as in cramming. Spaced study, also called spaced or distributed practice, puts time or other material between the repetitions.

It is one of the oldest and most consistent results in psychology. Ebbinghaus demonstrated it in 1885,^[2] and a 2006 meta-analysis, which pooled hundreds of experiments, found that only 12 of 271 comparisons of spaced and massed study showed no benefit from spacing, or a negative one.^[1]

How is the spacing effect different from spaced repetition?

The spacing effect is a finding about memory; spaced repetition is a study method that puts it to use. The effect says that spreading study out helps. Spaced repetition turns it into a schedule: you review each item at intervals that usually grow over time, often with flashcards in an app such as Anki.

Spaced repetition adds two things the spacing effect does not require. The first is testing yourself at each review, which strengthens the benefit of spacing.^[3] The second is intervals that grow; whether growing gaps beat equal ones is still unclear, but both beat massed study.^[2] For the evidence on the method itself, see does spaced repetition work?

Why does the spacing effect happen?

No one knows for sure: researchers have proposed several explanations, and none has won.^[1][4] The four main accounts are:

• Deficient processing. A repeat that comes straight after the first feels familiar, so you pay it less attention. This explains why massing is weak, but not why the best gap depends on when you are tested, and it was the only one of the four that the 2006 meta-analysis did not keep as a candidate.^[1]
• Encoding variability. Each session links the item to a slightly different context, such as your surroundings, mood and thoughts, which gives you more cues to bring it back later. But making people deliberately vary how they study often fails to help.^[1]
• Study-phase retrieval, or reminding. Seeing the item again makes you recall the earlier encounter, and that recall strengthens the memory, more so when it takes effort. After too long a gap, the earlier encounter can no longer be recalled, so there is a “sweet spot” in between.^[4]
• Consolidation. A memory becomes more stable in the time after learning, and a later repetition builds on that more stable memory, as long as it has not been lost.^[1]

Some theories combine several of these mechanisms.^[4] The idea that effortful recall helps memory also underlies retrieval practice and desirable difficulties.

Does a longer gap between sessions always help?

No: a longer gap helps up to a point, and that point depends on how long you need to remember the material. Researchers call the effect of gap length the lag effect.^[1] In a large study with final tests up to a year later, lengthening the gap between two sessions first raised final scores, then slowly lowered them, and the best gap grew with the delay before the test.^[5]

In that study, the best gap was a few days for a test a week later and several weeks for a test a year later.^[5] A gap that is somewhat too long costs less than one that is too short. For example intervals and exam plans, see the spaced repetition schedule.

Who and what does the spacing effect work for?

The spacing effect has been found across ages and many kinds of learning, although most studies test simple verbal material, such as word lists and facts, in the lab.^[2]

• Children. From preschoolers aged 3 or 4 to middle school students.^[2]
• Older adults. In a 2010 study, adults with an average age of 77 learned to recognise painters' styles better when the paintings were spaced out.^[6]
• Concepts and problem solving. Spacing has improved grammar learning and scores on new maths problems of a practised type.^[2][3]
• Motor skills. Surgical residents who spread four microsurgery training sessions over weeks did better a month later than those trained in one day.^[2]
• Classrooms. Spaced lessons or reviews improved vocabulary, history and reading in schools.^[2]

Studies of complex skills and real classrooms are fewer than lab studies of facts. See where the evidence is weaker.

Why does cramming feel more effective than it is?

Cramming feels effective because the benefit of spacing shows up later, not while you study. In a flashcard experiment, spacing beat massing for 90% of participants, yet after the first session 72% believed massing had worked better.^[7] Spacing also beat cramming, meaning all the study on the day before the test.^[7]

The same misjudgement held in the 2010 study: both young and older adults rated massing as better for learning painters' styles, although spacing worked better.^[6] Schools have been slow to use the effect too. A 1988 article called it “a case study in the failure to apply the results of psychological research”,^[8] and a 2012 review found it was still not built into curricula in any systematic way.^[2]

How can you use the spacing effect?

Spread your study time over several days, and come back to material after a gap instead of repeating it straight away. A few habits help:

• Split sessions. Study a topic for a shorter time on several days instead of once for longer.
• Use one large stack of flashcards. Splitting cards into small stacks brings each card back sooner; one large stack spaces them out.^[7]
• Test yourself at each review. Recalling the answer adds to the benefit of spacing.^[3] See active recall.
• Revisit older material. Start sessions with a short review of earlier topics, or use cumulative quizzes.^[2]
• Mix topics. Practising several topics in one session spaces out each of them; see interleaving.^[2]
• Let software track the gaps. Flashcard apps such as Anki schedule each card on its own, from your answers.