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An ordered setup from Mace's 1-2-4-8 day intervals through Leitner's flashcard boxes, including what to do when recall fails.
The method of spaced repetition was first conceived of in the 1880s by Hermann Ebbinghaus, who postulated that the forgetting curve can be curbed by reviewing information at several intervals over time. The intervals are not arbitrary: each successful recall is evidence that the memory can survive a longer gap.
In his 1932 book Psychology of Study, C. A. Mace proposed that acts of revision should be spaced in gradually increasing intervals, roughly one day, two days, four days, eight days, and so on. This is the simplest workable schedule and it needs nothing more than a calendar.
The Leitner system, a flashcard method where cards are reviewed at increasing intervals, was proposed by the German science journalist Sebastian Leitner in the 1970s. Cards answered correctly move to a slower box; cards answered incorrectly return to a faster box. The boxes make the expanding interval physical and visible.
On narrow screens, swipe or scroll the plate sideways.
Setting up a topic follows a fixed order. First, learn the material once and write the questions you will later answer from memory. Second, schedule the first review for the next day. Third, review and record which items failed. Fourth, return failed items to a shorter interval and let successful ones move out.
The most common failure at this stage is keeping intervals fixed. A fixed daily review feels disciplined but wastes time on items that no longer need it and starves the items that do. Expanding intervals spend the same minutes where they change retention.
The second common failure is skipping failed items to keep the schedule tidy. A failed recall is the schedule working: it identifies exactly the item that needs a shorter gap. Drop it back one interval and continue.
Further reading