What is spaced repetition?

Spaced repetition is a way of studying where you review information at growing intervals (a day later, then a few days later, then a week or more), timed for the moment you’re about to forget it. Each well-timed review makes the memory last longer, so you need fewer reviews over time.

The forgetting curve

In 1885 the German psychologist Hermann Ebbinghaus tested his own memory with lists of nonsense syllables. He found that memory drops steeply in the first hours and days after learning, then levels off. This shape is known as the forgetting curve.

He also found something more useful: every time you successfully recall something, the curve gets flatter. The memory fades more slowly, and the next review can wait longer.

Why spacing beats cramming

Cramming (reviewing the same material many times in one session), can get you through tomorrow’s quiz. But researchers have found again and again that spreading the same study time over several days leads to much better long-term memory. This is called the spacing effect, and it’s one of the most reliable findings in learning science.

Spacing works because of effort. When a memory has started to fade, pulling it back takes work, and that work strengthens it. Reviewing something you saw five minutes ago feels easy, and teaches you little.

Active recall: the other half

Spaced repetition works best with active recall: answering a question from memory instead of re-reading the answer. That’s why flashcards and spaced repetition go together so well. The front asks; you answer; the back checks you.

How the intervals grow

A typical schedule might look like this:

• Learn a new card today.
• Review it tomorrow.
• If you remember it, review again in about 3 days.
• Then about a week later, then a few weeks later.
• If you forget it, it comes back soon and the gaps start growing again.

The exact numbers matter less than the principle: remembered cards wait longer; forgotten cards come back sooner.

What the research says

The spacing effect is one of the most tested ideas in psychology. A 2006 review by Cepeda and colleagues brought together hundreds of experiments with word lists, facts and other material. Spreading study sessions apart beat cramming them together in almost every comparison.

A later study by the same group asked how long the gaps should be. The answer: the best gap grows with how long you want to remember something. If the test is next week, reviewing again after a day or two works well. If you want to know the words next year, the gaps should stretch to weeks.

In 2013, Dunlosky and colleagues rated ten popular study techniques. Only two earned their highest rating: practice testing (answering questions from memory) and distributed practice (spacing). Rereading and highlighting, the two most common habits, were rated low. Flashcards with a review schedule combine both of the winners.

From paper boxes to algorithms

In the 1970s, the German science writer Sebastian Leitner described a simple way to space reviews on paper. Cards live in a row of boxes. A card you answer correctly moves to the next box, which you review less often. A card you miss goes back to the first box. It works, but you have to move every card by hand and keep track of when each box is due.

Software removed that bookkeeping. SuperMemo, written by Piotr Woźniak in the late 1980s, scheduled every card on its own instead of every box. Its SM-2 algorithm became the basis for many apps, including Anki, which in 2023 also added a newer scheduler called FSRS. Toly follows the same principle: each card gets its own next review, based on your answers.

How to start today

1. Keep sessions short. Ten minutes a day beats an hour on Sunday.
2. Add a few new items a day. Reviews grow as your deck grows, so start small.
3. Be honest. If you hesitated or got it wrong, mark it as forgotten. The schedule depends on it.
4. Make cards specific. One idea per card is easier to recall than a paragraph.
5. Don’t skip the hard ones. The cards that feel difficult are the ones doing the most work.

Common mistakes

• Adding too much at once. Every new card brings future reviews with it. Fifty new cards today means a heavy week ahead.
• Grading yourself kindly. If the answer came slowly or only half right, mark it as missed. A generous grade pushes the card too far into the future.
• Skipping a week, then cramming. Missing a day is fine. Come back and clear what’s due a little at a time rather than all at once.
• Stopping at recognition. Recognizing a word when you see it is easier than recalling it when you need it. Say the answer before you reveal it.

Doing it without the admin

Doing spaced repetition by hand (tracking which card is due when), is tedious. Apps do the timing for you. In Toly, you only answer Again or Got it, and every card is scheduled automatically. Toly also writes the cards for you: type a word or a concept, and the back is ready in a second.

Remember it right before you’d forget.

That’s the whole idea. Start with a handful of cards today, review for a few minutes tomorrow, and let the intervals do the rest.

Sources

• Ebbinghaus, H. (1885). Über das Gedächtnis. Leipzig: Duncker & Humblot.
• 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. doi:10.1037/0033-2909.132.3.354
• 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. doi:10.1111/j.1467-9280.2008.02209.x
• 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. doi:10.1177/1529100612453266