Spaced Repetition — When to Review

You now know that active recall is the most effective way to study, and that the forgetting curve means you will lose most of what you learn unless you review it. The next question is: when should you review?

The answer is one of the most powerful ideas in all of learning science: spaced repetition.

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The Spacing Effect

In 1885, Ebbinghaus did not just discover the forgetting curve. He also discovered something equally important: distributing study sessions over time produces far better retention than massing them into a single session, even when the total study time is identical.

This is called the spacing effect, and it has been replicated in over a thousand studies across every age group, every subject, and every type of material.

Here is a simple example:

Approach	Total Study Time	Retention After 1 Week	Retention After 1 Month
Massed: 3 hours on Sunday	3 hours	~35%	~15%
Spaced: 1 hour on Monday, Wednesday, Friday	3 hours	~60%	~45%