Spaced Repetition

Spaced repetition is the practice of reviewing information at increasing intervals over time, instead of cramming it all at once. You study something today, see it again in a few days, then in a week or two, then in a month, and so on. Each successful recall buys you a longer interval before the next review. The technique is built directly on top of one of the oldest and most reliable findings in the science of memory: the forgetting curve.

In the 1880s, the German psychologist Hermann Ebbinghaus tested his own memory using lists of nonsense syllables and carefully tracked how quickly he forgot them. He found that forgetting is steep and fast at first, then levels off: you lose a large fraction of new material within hours and days, and what survives that early drop tends to stick longer.

Crucially, he also noticed that relearning something took less effort than learning it the first time, and that spreading study sessions out across days produced better retention than packing them together. That second observation is the spacing effect, and spaced repetition is simply the deliberate, scheduled use of it.

A review that comes too late means you have already forgotten and have to relearn from scratch. Spaced repetition aims for the productive middle.

This chapter teaches you to do that by hand with a Leitner box, and then explains how software such as Anki automates the same logic. Spaced repetition does not make you smarter or raise your IQ. What it does is let you organize, encode, and recall a large body of facts far more efficiently than rereading ever could, with less total study time spread across more days.

Use spaced repetition whenever you need to retain a large set of discrete facts in durable memory over weeks, months, or years rather than for a single afternoon. It shines for material that is item-based and clearly testable: a prompt on one side, an answer on the other.

It is the wrong tool for understanding a difficult argument or developing a skill that requires practice rather than recall, though it pairs well with those once you have isolated the facts inside them. If your test is tomorrow, spacing has little room to operate and you should focus on active recall and sleep. Spaced repetition is the long game.

Suppose you are learning the chemical symbols of ten elements. The tricky ones, Na, K, Fe, Au, Ag, Pb, Sn, come from Latin names, so they resist memorization. Run the box yourself below: answer each due card from memory, flip it, mark it honestly, then move to the next day and watch where the cards go.

Notice what happens: the easy symbols climb a box every time they come due and soon only resurface every few days, while the one you keep missing (for most people, tin) gets drilled day after day until it sticks. The system spends your attention precisely where it is needed.

Now scale up. You are preparing for a Spanish proficiency test ten weeks away and need to learn 600 vocabulary items plus 40 irregular verb conjugations. Cramming 640 cards is hopeless; spacing makes it routine. Set a sustainable intake of roughly 15 new cards per day. Your daily workload stays small because most cards quickly climb out of the daily and every-other-day boxes.

This is also where Anki's logic improves on rigid boxes. Instead of fixed box intervals, Anki computes a per-card interval and rates each answer Again, Hard, Good, or Easy. A “Good” answer multiplies the previous interval by an ease factor (around 2.5 by default), so intervals grow smoothly. “Again” collapses the interval, like demotion to Box 1; “Easy” jumps it further ahead.

The research backing this approach is robust: Cepeda and colleagues, in a 2006 review of hundreds of spacing experiments, found that spaced study reliably beat massed study, and that wider gaps favor longer-term retention, while Roediger and Karpicke showed in 2006 that the act of testing yourself, the retrieval each review demands, is itself a powerful memory builder.

Setting this up takes about fifteen minutes; running it takes a few minutes a day.