The science of learning, explained

Spaced repetition is a study technique that spaces your reviews out over increasing intervals, timing each one for the moment you are about to forget. It is the single most evidence-backed way to move facts into long-term memory with the least total study time.

Active recall means retrieving an answer from memory instead of re-reading or recognising it. The effort of retrieval is what strengthens the memory — which is why being tested beats reviewing notes, an effect researchers call the testing effect.

The forgetting curve describes how quickly we lose newly learned information: memory decays fastest in the first hours and days, then more slowly. Each well-timed review flattens the curve, so the same fact is forgotten more and more slowly over time.

The Leitner system is a manual spaced-repetition method: flashcards live in numbered boxes, and a card you recall correctly graduates to a slower-reviewed box while a card you miss drops back to the first. It is the simple, physical ancestor of every modern spaced-repetition algorithm.

Interleaving means mixing different problems or topics within one study session rather than practising them in separate blocks. It feels harder than blocked practice, but it produces better long-term retention and a stronger ability to tell similar things apart.

Cloze deletion is the fill-in-the-blank flashcard format: a sentence is shown with one key word hidden, and you must recall the missing piece in context. Keeping the surrounding context makes the card easier to learn and the memory easier to use in real situations.

SM-2 is the original spaced-repetition algorithm from the 1980s; FSRS (the Free Spaced Repetition Scheduler) is its modern, data-driven successor. FSRS predicts memory more accurately, so it schedules fewer reviews for the same retention — and it is the algorithm EverFlip uses.