Spaced Repetition — When to Review

You now know that active recall is the most effective way to revise and that the forgetting curve causes rapid memory decay. The next question is: when should you review? The answer is the second most powerful evidence-based technique in the revision toolkit: spaced repetition.

What Is Spaced Repetition?

Spaced repetition is the practice of reviewing material at gradually increasing intervals rather than massing all your study into a single session. Instead of studying photosynthesis for three hours on Monday and never returning to it, you study it for 30 minutes on Monday, 15 minutes on Wednesday, 10 minutes on Saturday, and 10 minutes the following Thursday.

The total study time may be similar — or even less — but the retention is dramatically higher.

The Spacing Effect

The underlying phenomenon is called the spacing effect, first documented by Ebbinghaus in the 1880s and confirmed by over a century of subsequent research. The spacing effect states that:

Information reviewed at spaced intervals is retained far better than the same information reviewed in a single massed session, even when total study time is equal.

This is one of the most replicated findings in cognitive science. It works across all ages, all subjects, and all types of material.

Why Spacing Works

Spacing works because of the relationship between forgetting and retrieval strength. When you review material after some forgetting has occurred, the retrieval is harder — and harder retrieval produces stronger memory reinforcement.

flowchart TD
    A[Learn material for the first time] --> B[Memory begins to decay<br/>Forgetting curve]
    B --> C{Review before<br/>complete forgetting}
    C -->|Yes| D[Effortful retrieval<br/>= stronger memory]
    D --> E[Forgetting curve resets<br/>but shallower this time]
    E --> F[Wait longer before<br/>next review]
    F --> C
    C -->|Too late - completely forgotten| G[Must re-learn from scratch<br/>Little benefit from first learning]
    C -->|Too early - still fresh| H[Easy retrieval<br/>= weak reinforcement]
    H --> I[Wasted review — <br/>memory barely strengthened]

The ideal moment to review is when the material is starting to fade but has not completely disappeared — when retrieval requires genuine effort but is still possible. This is sometimes called the zone of desirable difficulty.

If you review too early (while everything is still fresh), the retrieval is effortless and does little to strengthen the memory. If you review too late (after complete forgetting), you are essentially learning from scratch and lose the cumulative benefit.

Optimal Spacing Intervals

Research suggests that optimal spacing intervals roughly follow an expanding schedule. While the exact intervals depend on factors like the complexity of the material and your individual forgetting rate, a widely supported general schedule looks like this:

Review Number	Approximate Interval	Practical Example
1st review	1 day after initial learning	Monday: learn it. Tuesday: first review.
2nd review	3 days after 1st review	Friday: second review.
3rd review	7 days after 2nd review	Following Friday: third review.
4th review	14-21 days after 3rd review	Two to three weeks later: fourth review.
5th review	30+ days after 4th review	A month later: fifth review.

After five well-spaced reviews, most material will be firmly embedded in long-term memory and will require only occasional maintenance.

A Worked Example for a Specific Topic

Suppose you are studying ionic bonding for Chemistry on Monday 1st April:

Date	Action	Time
Mon 1 Apr	Initial study — notes, textbook, understanding	30 min
Tue 2 Apr	First review — blank page recall + flashcards	15 min
Fri 5 Apr	Second review — practice questions on ionic bonding	15 min
Fri 12 Apr	Third review — mixed quiz including ionic bonding	10 min
Fri 26 Apr	Fourth review — past exam questions on bonding	10 min

Total time: approximately 80 minutes spread over nearly a month. Compare this with a student who spends 80 minutes on ionic bonding the night before the exam and retains far less.

Spaced Repetition vs. Cramming: The Evidence

The Cepeda et al. (2006) Meta-Analysis

This landmark analysis combined results from 254 separate studies involving over 14,000 participants. The consistent finding: spaced practice produced significantly better long-term retention than massed practice in every single comparison. The average improvement was 10-30% on delayed tests.