Spaced Repetition and the Leitner Box

1. What You're Learning

Spaced repetition means reviewing material several times with growing gaps between reviews, instead of all at once. You will learn why it works, the simplest way to schedule it by hand (the Leitner box), and how software schedulers extend the same idea.

2. Why This Matters

Most of what we read fades quickly. In the 1880s Hermann Ebbinghaus measured his own forgetting of lists of nonsense syllables. He found that it is steep at first and then levels off, a shape now called the forgetting curve. Later research on distributed practice has repeatedly found that the same total study time produces better long-term retention when it is spread out rather than massed together.

For a curriculum that keeps growing, this is the difference between a library you have read and a library you remember.

3. The Idea

Two observations combine:

• Memories fade unless used. The longer since you last recalled something, the more likely you are to fail to recall it.
• Successful recall makes the memory more durable. After a successful review you can wait longer before the next one.

So a good schedule reviews an item shortly before you would forget it. If you remember it, the gap grows. If you forget, the item goes back to short gaps. Easy items quickly stop costing time, and hard items get the attention.

The Leitner system, described by Sebastian Leitner in 1972, implements this with flashcards and a few boxes. Each box has a review interval that grows from box to box. A card you answer correctly moves one box forward. A card you miss goes back to Box 1.

4. Walk Through an Example

Use three boxes: Box 1 is reviewed every day, Box 2 every 3 days, Box 3 every 5 days. A new card starts in Box 1. The rule for the next review date is today + the interval of the box the card lands in.

Look at what happened on day 9. The gap had grown to five days, which was too long for this card. The miss sent it back to daily review, and two quick successes rebuilt the gap. A card you always get right would only be seen on days 1, 4, 9, 14, 19 and so on, so it costs less and less time.

Modern apps replace the fixed boxes with a model that estimates, for each card, how likely you are to recall it today. The open-source FSRS scheduler, for example, fits that model to your own review history and schedules each card for when your predicted recall drops to a target you choose. The logic is the same: long gaps after successes, short gaps after failures.

5. Sources to Explore

• Forgetting curve (Wikipedia): Ebbinghaus's self-experiments with nonsense syllables and later replication attempts.
• Cepeda et al. (2006), “Distributed practice in verbal recall tasks: A review and quantitative synthesis”, Psychological Bulletin 132(3): a large review of research on spacing.
• Leitner system (Wikipedia): the box method, including the three-box schedule used above.
• open-spaced-repetition/fsrs4anki: an open-source (MIT) scheduler and optimiser for the Anki flashcard app. You do not need to install it to follow this lesson.