Spaced repetition means reviewing material at increasing intervals: a day after you learn it, then a few days later, then a week, then a month. Each review comes just as you're starting to forget, which makes the memory stronger and the next gap longer. It's one of the most reliable findings in the science of learning, and one of the least used by students.
The forgetting curve
In the 1880s the German psychologist Hermann Ebbinghaus memorised lists of nonsense syllables and tested himself at different delays. He found memory falls off steeply at first and then levels out, a pattern now called the forgetting curve, and that each review flattened the curve: the material was lost more slowly the next time. Later research has confirmed the shape many times over.
The spacing effect
The same total study time, spread out, beats the same time crammed into one session. The effect has been found for vocabulary, facts, maths, motor skills and more. A large study by Nicholas Cepeda and colleagues in 2008 tested gaps between study sessions against how long people needed to remember the material, and found the best gap grows with the retention time: roughly 10 to 20% of it. For an exam in a month, review every few days; for knowledge you want for a year, every few weeks.
A simple schedule
| Day 1 | Learn it, then test yourself at the end of the session |
|---|---|
| Day 2 | Review: recall first, then check |
| Day 4 | Review again |
| Day 8 | Review |
| Day 16 | Review |
| Day 30, then monthly | Review whatever you still get wrong |
Roughly doubling the gap each time is a good rule. If you get something wrong, start its gaps again from the beginning.
Tools for spacing
- The Leitner system: flashcards in numbered boxes, devised by the German writer Sebastian Leitner in the 1970s. Cards you get right move up a box and are reviewed less often; cards you get wrong go back to box one.
- Spaced repetition software, such as Anki or SuperMemo, which schedules each card for you based on how well you knew it.
- A calendar. For topics rather than cards, simply put the review dates in your calendar when you first study something.
Spacing and recall, together
Spacing works best when each review is a test, not a reread: try to recall first, then check. That's active recall, and the combination is about as effective as studying gets. A good daily session might open with a Pomodoro of reviews that are due, then move on to new material. Plan the day's sessions with the Pomodoro calculator and run them with the Pomodoro timer.
Why it feels wrong
Cramming feels productive: the material is fresh, and you can recite it at the end of the evening. Spaced review feels worse, because you've forgotten some of it each time. That struggle is exactly what makes it work. Psychologists call these “desirable difficulties”: harder in the moment, better in the exam.
A little every day
Spaced repetition rewards short, regular sessions. In Flight, make your daily review a short hop: 30 to 60 minutes of recall before you move on to anything new.