Spaced Repetition & Long-Term Memory

What is spaced repetition?

Spaced repetition schedules review of material at increasing intervals, exploiting the spacing effect to maximise long-term retention. Instead of re-reading notes daily, you review each item just before you are likely to forget it. This produces far better retention per hour of study than any massed practice approach — and is the basis for tools like Anki.

What is the Ebbinghaus forgetting curve?

The forgetting curve (Ebbinghaus, 1885) shows memory decays exponentially after learning: roughly 40–60% is forgotten within 1 hour, 60–75% within 24 hours without review. Each review resets the curve at a higher baseline, producing longer and longer retention intervals. Spaced repetition schedules reviews to catch material just before the threshold of forgetting.

How does the SM-2 algorithm work?

SM-2 assigns each card an interval and an ease factor. After each review, you rate recall 0–5. Ratings of 4–5 multiply the interval by the ease factor; ratings of 0–2 reset the interval to 1 day. Cards you find hard are reviewed more often; easy cards less often — automatically allocating study time to where your memory is weakest.

How is spaced repetition different from reviewing notes?

Reviewing notes is a passive re-exposure strategy — you see the information again, which creates a feeling of familiarity but minimal long-term retention. Spaced repetition with active recall requires you to retrieve the information from memory before seeing the answer, and schedules that retrieval at the optimal moment to interrupt forgetting. The combination is far more efficient.