The forgetting curve, the starting point
In 1885, German psychologist Hermann Ebbinghaus published the first experimental measurements of human forgetting. Self-testing on nonsense syllables, he showed that without review, you forget about 50% of the content within 24 hours, 70% within a week, 90% within a month. The forgetting curve is exponential, not linear.
This finding stayed a long-time academic curiosity. But Ebbinghaus also observed that a review just before the memory trace fades considerably strengthens memory for next time. That's the spaced repetition effect.
Why it works neurologically
When you memorize information, your brain creates a fragile memory trace. Without reactivation, the trace fades. A review reactivates it, but timing matters enormously. Review too soon and there's no benefit (the trace didn't need reactivation). Review too late and you're starting over.
The sweet spot is just before forgetting: the trace is fragile but still there. Retrieving it in that state strengthens neural consolidation, and the trace becomes more durable. Each cycle the optimal interval grows.
SuperMemo, ancestor of modern tools
In the 1980s, Polish researcher Piotr Wozniak turned Ebbinghaus's idea into an actionable algorithm. His SuperMemo (then SM-2 in 1987) calculates for each card the optimal next review based on your past responses. SM-2 became the standard, used by Anki and most flashcard apps.
FSRS-5, the modern evolution used by Diane
FSRS (Free Spaced Repetition Scheduler) is a more recent algorithm, designed in 2022 and refined up to FSRS-5 in 2024. It improves SM-2 on several fronts: finer modeling of the individual forgetting curve, accounting for intrinsic card difficulty, adaptation to your personal history.
Concretely, FSRS-5 predicts for each card the probability you'll remember it at a given instant, and surfaces it when that probability drops to a target threshold (typically 90%). You can tune this threshold to review more or less frequently.
How much time per day
Rule of thumb: 10 to 30 minutes a day are enough to maintain a deck of several hundred cards. Early on, you see more cards (all new). Over weeks, the rate stabilizes as mastered cards space out.
On a 500-card deck in learning phase, expect 20-30 minutes a day for 2-3 weeks, then 10-15 minutes a day to maintain.
Limits of the method
Spaced repetition works well for factual memorization: definitions, dates, vocabulary, formulas, classifications. It's less effective for deep understanding and practical skills (problem-solving, writing, speaking). For those, combine spaced repetition with practice exercises.
That's why Diane combines flashcards (memorization) with quizzes (active application). Both modes together cover retention and use of knowledge.