The Science of Spaced Repetition: Why It Works and How to Use It

The Forgetting Curve

In 1885, Hermann Ebbinghaus discovered that we forget new information in a predictable, exponential pattern. Within 24 hours of learning something, we forget approximately 70% of it. Within a week, we retain less than 25%.

But each time we review the information at the right moment (just before we would forget it), the forgetting curve flattens. The memory becomes more durable with each correctly-timed review.

How Spaced Repetition Works

Instead of reviewing all material equally, spaced repetition algorithms schedule each item for review at the optimal interval:

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1. You learn a new word. Review it tomorrow.
2. You remember it correctly. Review it in 3 days.
3. Correct again. Review in 1 week.
4. Correct again. Review in 2 weeks.
5. Correct again. Review in 1 month.

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If you forget the item at any point, the interval resets to a shorter duration. Items you find easy are reviewed less frequently. Items you find difficult are reviewed more often.

The Evidence

A 2019 meta-analysis in the Journal of Educational Psychology analyzed 29 studies involving over 3,000 participants and found that spaced repetition produced significantly better long-term retention than massed practice (cramming) across all domains tested.

For language learning specifically, a study by Bahrick and Phelps (1987) found that spaced practice sessions produced retention that lasted decades, while massed practice showed near-complete forgetting within years.

How to Implement It

Anki (Free, Cross-Platform)

The gold standard for spaced repetition. Create flashcards for vocabulary, grammar patterns, or phrases. Anki schedules reviews automatically using a sophisticated algorithm.

Tips for effective Anki use:

• Keep cards simple: one concept per card
• Add context: include an example sentence, not just the word
• Use images and audio when possible
• Do your reviews every day without exception
• Limit new cards to 10-20 per day to avoid review pile-up

Manual Spaced Repetition

If you prefer physical flashcards, use the Leitner Box System. Create 5 boxes. New cards start in Box 1 (review daily). Correct answers move to Box 2 (review every 3 days), then Box 3 (weekly), Box 4 (biweekly), Box 5 (monthly). Incorrect answers return to Box 1.