Space reviews at increasing intervals after each successful retrieval

After each successful review, increase the interval before the next one — this is how you make memories last for years on the same total time investment.

Why it works

Each successful retrieval resets the forgetting curve at a higher baseline — the slope of the subsequent forgetting is shallower than before. This means the next review can be delayed longer while still catching the material before it degrades below a useful threshold. Expanding intervals over time means total review time falls while retention increases, a compounding return that does not occur with fixed-interval or random review schedules.

How to do it

1. After a successful first review, wait approximately twice as long before the second (e.g., 1 day → 2 days → 4 days).
2. If retrieval fails at any interval, treat it as a new item and restart the schedule from 1 day.
3. Use a flashcard app (Anki uses a spaced repetition algorithm) or a simple calendar-based schedule for explicit tracking.
4. Calibrate the expansion multiplier down if you have a near-term use-it-or-lose-it deadline, up if you need lifelong retention.

Evidence

The spacing effect is among the most replicated findings in all of memory research. Cepeda et al.’s (2006) meta-analysis of 317 experiments confirmed large distributed-practice advantages. Spaced repetition algorithms (Leitner, SM-2) apply this finding computationally. (rct)

Optimal spacing ratios depend on the retention target period; the doubling heuristic is an approximation. For very long-term retention (years), the optimal gaps are much longer than simple doubling suggests.

Sources

• Cepeda et al. (2006), Distributed practice in verbal recall tasks, Psychological Bulletin
• Ebbinghaus, H. (1885/1913), Memory: A contribution to experimental psychology

Common mistake

Using fixed weekly reviews rather than expanding intervals — fixed schedules over-review well-retained items and under-review fragile ones, wasting time on both ends.