Spaced Repetition: How It Works and How to Use It
· 3 min read
Spaced repetition is a way of reviewing material at increasing intervals: soon after you learn it, then a few days later, then a week or more later. Each review happens just as the memory starts to fade, which resets it and makes it last longer. It is how you remember a lot without studying it every day.
The forgetting curve
In the 1880s Hermann Ebbinghaus memorised lists of nonsense syllables and tested himself at different delays. He found memory dropped steeply in the first hours and days, then levelled off. That shape is now called the forgetting curve, and later research has broadly confirmed it.
The useful part is what happens after a review. Each time you successfully recall something, the curve flattens. The next drop is slower. So the gaps between reviews can grow.
Why spacing beats cramming
Cramming puts all the repetitions in one evening. It works for tomorrow and fails for next month. Spacing puts the same repetitions across days, so each one happens when recall takes a little effort.
A 2006 meta-analysis by Cepeda and colleagues, covering hundreds of experiments, found spaced practice beat massed practice across a wide range of materials and ages.
A simple spaced repetition schedule
You do not need an algorithm to start. A rough schedule like this works.
- Day 0: learn it and test yourself once.
- Day 1: review.
- Day 3: review.
- Day 7: review.
- Day 14 to 21: review.
- Miss it at any point: go back to a one-day gap.
Try this: Keep each day’s reviews short. If the queue is over 30 minutes, add fewer new cards for a few days rather than skipping reviews.
Spacing before an exam
Research suggests the best gap depends on how long you need to remember something. For an exam a month away, reviews a few days to a week apart work well. For an exam next week, gaps of a day or two.
So start early if you can. Spacing cannot happen in the last 48 hours.
Leitner boxes and apps
The Leitner system does this with paper: cards move to a later box when you get them right, and back to box one when you miss. Apps do the same thing automatically, which removes the bookkeeping and the temptation to review only the easy cards.
Common questions
How many days apart should I space my reviews?
Start with one day, then three, then seven, then two to three weeks, and reset to a short gap whenever you miss. Spaced repetition apps adjust these gaps for each card based on how you grade it.
Can I use spaced repetition if my exam is next week?
Yes, with shorter gaps: review each topic on two or three separate days before the exam. It still beats doing it all on one night.
Is spaced repetition only for flashcards?
No. You can space quizzes, practice problems, and essay outlines. Flashcards are just the easiest format to schedule.
What happens if I miss a day of reviews?
Nothing breaks. The due cards wait. Do the oldest ones first, and add fewer new cards until the queue is back to a comfortable size.
Calm Study
Calm Study writes the flashcards from your notes and schedules them for you. Grade each card Again, Hard, or Good, and only the cards that are due come back, so each session is spent on what is slipping.