Question 1

What is spaced repetition?

Accepted Answer

Spaced repetition is a study technique that schedules review of material at increasing intervals over time, exploiting the spacing effect to maximise long-term retention. Instead of re-reading notes the night before an exam (massed practice), you review material multiple times at optimally-spaced intervals — typically 1 day, then 3–6 days, then 2 weeks, then a month. Each review session should be a retrieval test, not a re-read. Cepeda et al. (2006) found spaced practice produces 1.5–3× better retention than massed practice at delayed intervals.

Question 2

What is the Ebbinghaus forgetting curve?

Accepted Answer

The forgetting curve is a mathematical model of memory decay, discovered by Hermann Ebbinghaus in 1885. It shows that memory decays exponentially after learning: roughly 40–60% is forgotten within 1 hour, 60–70% within 1 day, and ~75% within a week — without review. Crucially, each review resets the curve at a higher, flatter baseline. After 3–4 well-timed reviews, the curve approaches flat and the memory becomes durable. Murre & Dros (2015) replicated Ebbinghaus's findings unchanged after 130 years.

Question 3

How does the SM-2 algorithm work?

Accepted Answer

SM-2 (developed by Piotr Wozniak, 1990) assigns each flashcard an interval and an ease factor. After each review, you rate recall on a 0–5 scale. The next interval = current interval × ease factor. Cards rated as easy get a higher ease factor (longer intervals); cards rated as hard get a lower ease factor (shorter intervals, more frequent review). This adaptive schedule automatically allocates more review time to weak items. Anki implements SM-2 with modern modifications and is free and open-source.

Spaced Repetition & Long-Term Memory

The Forgetting Curve — Ebbinghaus's Discovery

The Spacing Effect — Why Distribution Beats Cramming

The Leitner Box — Analogue Spaced Repetition

How Spaced Repetition Software Works — The SM-2 Algorithm

Card Design — The Minimum Information Principle

Building Your Spaced Repetition System