Spaced Repetition: The Science Behind Anki and Why Timing Your Review Changes Everything

Spaced repetition is the scheduling of reviews at increasing intervals to catch information just before it would be forgotten — the timing is what makes it dramatically more effective than massed practice. It is the algorithm behind Anki, the most widely used learning software among medical students, language learners, and anyone else whose learning requires retaining large amounts of information over time. Here is the complete guide to understanding the science and applying it effectively.

The Forgetting Curve and Why Timing Matters

Hermann Ebbinghaus's forgetting curve — documented in the 1880s and replicated consistently since — shows that memory for new information decays rapidly in the hours and days after learning, then more slowly over subsequent weeks and months. Without any review, approximately 50% of new information is forgotten within a day, and 80% within a week. The critical insight: reviewing information just before you would forget it produces the greatest strengthening of that memory per review time invested. Reviewing too soon (when the memory is still strong) wastes time because the memory did not need strengthening. Reviewing too late (after forgetting has already occurred) means you are essentially relearning rather than strengthening. Spaced repetition algorithms calculate the optimal review timing for each piece of information based on your past performance with it.

How Anki Works

Anki is free flashcard software that implements the SM-2 algorithm (a well-validated spaced repetition algorithm) to schedule your reviews. When you review a card, you rate how easily you recalled it (Again, Hard, Good, Easy). Anki uses this rating to calculate when to show you the card next — cards you recalled easily are pushed further into the future; cards you struggled with are shown again soon. Over time, easy cards might appear once a month or once a year while difficult cards appear more frequently. The result: you spend your review time proportionally on what you actually need to practice, not on what you already know well.

Building an Effective Anki Practice

The single most important Anki principle: do your reviews every single day without exception. Missing days causes card accumulation — when you return after missing three days, you face three days of due reviews simultaneously, which creates a psychologically overwhelming pile that causes many people to abandon the habit entirely. The daily review habit, even for just 10-15 minutes, is more valuable than occasional long review sessions. Card quality: one simple, specific thing per card — not a paragraph of information. The card should test one fact, one concept, or one language item. Cards that test too much at once produce weak, ambiguous answers that the algorithm cannot optimize well.

Honest Bottom Line: Spaced repetition works by reviewing information at the optimal moment just before forgetting — too soon wastes time, too late means relearning. Anki implements this with SM-2 algorithm that personalizes review intervals based on your recall performance for each card. Daily review consistency matters more than session length — missing days creates card accumulation that causes most people to abandon the habit. One specific fact per card, never paragraphs — cards that test too much produce ambiguous performance data that the algorithm cannot optimize. Combined with active recall, spaced repetition is the highest-evidence learning technique available.