Spaced repetition: review just before you forget

The forgetting curve

In 1885 the German psychologist Hermann Ebbinghaus published experiments on his own memory. He learned lists of nonsense syllables and tested how much he kept over time. He found that forgetting is fastest soon after learning and then slows down. This shape is often called the forgetting curve.

Later research added the encouraging part: each time you successfully recall something, it fades more slowly afterward. The plan below shows that pattern with a simple model. It is a picture of the idea, not real data about your memory.

Why spacing works

Reviewing when an idea has started to fade makes recall a little harder, and that effort strengthens the memory. Reviewing again 10 minutes after studying feels easy and adds little. Spacing also gives you several chances to notice what you misunderstood, instead of one long session where everything blurs together.

Review plans you can copy

Exact intervals matter less than the pattern: short gaps first, then longer ones. Adjust to your calendar.

For the two-week plan, each review should be active recall, not rereading: flashcards, practice questions or a blank page. The day before the exam, focus on what you still miss.

The Leitner box system

The Leitner system was described by the German science journalist Sebastian Leitner in the 1970s. It schedules flashcards using a few boxes or piles.

1. Set up three to five boxes. Box 1 is reviewed every day, box 2 every few days, box 3 once a week and so on.
2. All new cards start in box 1.
3. When you answer a card correctly, move it up one box. It will come back less often.
4. When you miss a card, move it back to box 1, no matter where it was.
5. Cards that reach the last box are well learned. Review them now and then before a big exam.

The system puts your time where it is needed. Easy cards drift away and hard cards keep coming back. Many flashcard apps use a similar idea and schedule reviews for you, but paper boxes work just as well.

Mix topics in a session

When you review, try mixing related topics instead of doing all of one type at once. In math, for example, do a few problems from each of the last three chapters in random order. This is called interleaving. It is harder because you have to decide which method each problem needs, and that decision is often what a test checks.

Getting started this week

• After each class, write five to ten flashcards or cue questions from your notes.
• Review them the next day, then again in about three days, then in about a week.
• Put the reviews on a calendar or in your weekly plan so they don't depend on memory.
• Keep sessions short. Fifteen minutes of spaced review often does more than an hour of rereading.

Spaced repetition questions