Spaced repetition schedule: how review intervals grow

The idea

Spaced repetition reviews information just as you are about to forget it. Each successful recall makes the memory more durable, so the next gap can be longer: a day, then a few days, then weeks and months.

A large review of research on distributed practice found that spreading study over time improves long-term retention compared with massing it into one session.

A worked example

Suppose a card is first learnt today. With a simple doubling rule after a one-day start, successful reviews fall after 1, 2, 4, 8 and 16 days. The fifth review is on day 1 + 2 + 4 + 8 + 16 = 31.

Modern schedulers vary the growth per card. Easy cards stretch faster, hard cards slower, and FSRS adjusts the gaps based on your own history rather than a fixed multiplier.

What happens when you forget

When you press Again on a review, the card enters relearning and returns soon, often the same day. Its next interval after relearning is shorter than before, reflecting the lapse.

Frequent lapses on the same card usually mean the card needs rewriting. Split it, add context or connect it to an example.

When a paper schedule is enough

For a short list, such as 30 words for next week’s test, you can schedule reviews yourself: today, tomorrow, in three days and the day before the test. A calendar and a stack of cards are enough.

For hundreds of cards over months, an app is easier. It tracks every card’s interval so you only see what is due.

Try it yourself

Using the doubling example starting at 1 day (1, 2, 4, 8 days), on which day after learning does the fourth review fall?

References

• Cepeda et al. (2006), Distributed practice in verbal recall tasks, Psychological Bulletin
• Anki Manual: deck options, daily limits and FSRS