Spaced repetition

A 2006 review found 839 tests of spacing in 317 experiments. The same total study time works better split into sessions with gaps between them.

Two students each spend three hours on the same vocabulary list. One does all three hours the night before the test. The other does an hour on Monday, an hour on Wednesday and an hour on Friday. On a test the next morning they might score about the same. A month later the second student will usually remember far more. This is the spacing effect, also called distributed practice, and it is one of the most repeated findings in the study of learning.

How strong is the evidence?

In 2006 Nicholas Cepeda and colleagues pooled the research on verbal memory: 839 assessments of distributed practice in 317 experiments from 184 articles. Spaced study beat massed study, and the gap that gave the best retention grew longer as the final test moved further away. In 2008 the same group ran a large study to find the best gap. More than 1,350 people learned a set of obscure facts, reviewed them once after a gap of up to 3.5 months, and took a final test up to a year later. For any given test date, lengthening the gap first helped and then slowly hurt. The best gap grew as the test moved further away, but as a share of the test delay it shrank: about 20 to 40 percent of a one-week delay, and about 5 to 10 percent of a one-year delay.

Why would a gap help? One common explanation links back to the forgetting curve. A review that comes after some forgetting makes you work to pull the memory back, and that effort strengthens it more than rereading something still fresh. OpenStax adds a simpler reason: consolidation takes time, and spreading study out gives memories that time. Cramming feels productive because the material is fresh during the session, which is exactly why it misleads.

Boxes that schedule the gaps for you

Planning gaps by hand for hundreds of facts is tedious, so people use boxes. In the Leitner system, named after the German writer Sebastian Leitner, every card starts in the first box. Answer it correctly and it moves to the next box, which is reviewed less often. Miss it and it goes back to the first box. Easy cards drift toward long gaps and hard cards keep coming back, so your time goes where the forgetting is.

You can check the schedule with simple sums. A new card you get right every time is seen on day 0, then 1 day later (day 1), then 3 days later (day 4), then 7 days later (day 11), then 14 days later (day 25), and after that it waits 30 days, until day 55. Five short reviews spread over almost two months. These are nominal intervals: the code makes a review due one hour early. Try the cards now. A new card marked as known is due again about a day later; a missed card is due immediately. Cards already in higher boxes can wait longer.