Spaced repetition: how to beat the forgetting curve

In the 1880s the German psychologist Hermann Ebbinghaus carried out an unusual experiment on himself. He memorised lists of nonsense syllables, such as "DAX" or "BUP", and then measured how many he remembered after minutes, hours and days. He published his results in 1885.

The forgetting curve

Ebbinghaus noticed that forgetting was fastest at the start: a large part of the material was lost within the first hours and days, then forgetting slowed. This pattern came to be called the forgetting curve. He also noticed something else important: when he reviewed the lists over several separate days, he needed less time in total to remember them than when he repeated them many times on a single day.

What later studies showed

This effect, called the spacing effect or spaced repetition, is one of the most robust findings in the psychology of learning. In 2006 Nicholas Cepeda and colleagues combined the results of hundreds of experiments. The conclusion: when the same study time is divided into several spaced sessions, people remember more than when they cram it into a single session.

Another study by the same group, in 2008, asked how far apart reviews should be. The answer depends on how long you want to remember the material. As a rough rule, if you want to remember it for several months, the first review should be a few days or a week after the initial learning, not the next day and not after three months.

Reading for eight hours the night before an exam gives the feeling of knowing a lot. Spaced review gives knowledge that lasts.

Why it works

One explanation is that, when some time has passed, the memory becomes a little harder to retrieve. The effort to bring it back to mind strengthens it more than reviewing while it is still "fresh". This is closely linked to the testing effect: testing yourself is more useful than rereading.

A simple plan

1. Day 1: learn the material and test yourself once at the end.
2. Day 2 or 3: a short review, trying to recall before looking at your notes.
3. After a week: a second review, only of the parts you did not recall.
4. After three to four weeks: a third review.
5. For foreign-language vocabulary, use cards or apps that schedule words according to how well you know them.

Spaced repetition takes a little planning, but it saves time. Instead of learning the same thing again from scratch, you simply keep it alive.