Spaced repetition: what the intervals are actually for
Spaced repetition means reviewing material at growing intervals — a day later, then three, then a week, then a fortnight — instead of in one long session. The same total hours, spread differently, produce substantially more retention.
You do not need to remember the intervals. You need to review just as the material starts to slip.
Why gaps beat repetition
Reviewing something you still remember perfectly teaches you almost nothing. Reviewing something you have half-forgotten, and reconstructing it, is what strengthens the memory. The interval exists to let a controlled amount of forgetting happen.
This is why cramming works for Friday and fails by the following term: five reviews in one evening are five reviews of something you have not had time to forget.
The intervals, concretely
A workable schedule is: same day, next day, three days, one week, two weeks, one month. If you can produce the answer easily, push the next review further out. If you cannot, pull it back in.
The precise numbers are not sacred. Every scheduling algorithm in every flashcard app is an approximation of the same idea — review it when you are about to lose it — and the difference between a good schedule and a rough one is much smaller than the difference between spacing and not spacing.
Doing it without an app
Three piles. Everything starts in pile one, reviewed daily. Anything you get right twice moves to pile two, reviewed weekly. Anything you get right twice in pile two moves to pile three, reviewed monthly. Anything you get wrong drops back to pile one.
That is the Leitner system, it predates every app that implements it, and it works. The advantage of software is that it does the bookkeeping; the disadvantage is that setting it up becomes a project, and a project is easy to do instead of studying.
What it is bad at
Spaced repetition is excellent for facts, vocabulary, definitions, formulas and anything with a clean question-and-answer shape. It is poor for understanding — you cannot flashcard your way to grasping why a proof works, and trying produces a deck of memorised steps that collapses when the exam changes the question.
The honest split: use it for the material that has to be at your fingertips, and use worked problems and explanation for the material that has to be understood. Most courses need both, in different proportions.
Starting it late
If the exam is in ten days, the ideal schedule is not available and pretending otherwise wastes the ten days. Compress it: cover everything once, immediately, then review the material you got wrong on days two, four and seven, and leave the material you produced easily alone.
Spacing with three reviews beats one long session even inside a short window. The method degrades gracefully, which is more than can be said for cramming.
FAQ
What is spaced repetition?
Reviewing material at increasing intervals rather than all at once — typically a day, then a few days, then a week, then a fortnight. The gaps let a controlled amount of forgetting happen, and reconstructing the material is what strengthens it.
What are the best spaced repetition intervals?
Same day, next day, three days, one week, two weeks, one month is a workable default. Push an item further out when you can produce it easily and pull it back in when you cannot. The exact numbers matter far less than spacing at all.
Do I need an app for spaced repetition?
No. Three piles reviewed daily, weekly and monthly — the Leitner system — does the same job with index cards. An app handles the bookkeeping, which is useful at scale, but setting one up is also a very effective way to avoid studying.
Does spaced repetition work for understanding, not just facts?
Poorly. It is built for material with a clean question-and-answer shape: vocabulary, definitions, formulas, dates. Understanding comes from working problems and explaining ideas. Most courses need both, and using flashcards for the conceptual half is a common and costly mistake.
The part that usually breaks is not the method but the calendar around it. majored keeps courses, deadlines and exams together on Mac, iPad and iPhone, so a review schedule has a term to sit inside.