Spaced Repetition and the Forgetting Curve

Memory does not fade at a constant rate. Hermann Ebbinghaus, a German psychologist, documented this in the 1880s through self-experiments: within 20 minutes of learning new material, a significant portion is already forgotten. Within a week, most of it is gone unless something reinforces it.

The pattern follows an exponential decay curve — fast drop-off immediately after learning, then a slower decline.

Time since learning	Approximate retention (no review)
20 minutes	~58%
1 hour	~44%
1 day	~33%
1 week	~25%
1 month	~21%

These are approximate values based on Ebbinghaus's original research on arbitrary syllable memorisation. Retention for meaningful, connected material decays more slowly — but the curve still applies.

The core finding: without review, most new material is forgotten within days, regardless of how attentive you were during the lesson.

A single review does not defeat the forgetting curve — it resets it. Each time you successfully retrieve a piece of information, the decay curve starts again from a higher baseline. The material takes longer to forget after each successful review.

Spaced repetition is the practice of reviewing material at increasing intervals timed to catch it just before it would be forgotten. The intervals grow longer after each successful review because each retrieval strengthens the memory trace.

How the retention curve changes with spaced reviews:

Review	Approximate interval	Effect
1st review	1 day after learning	Retention resets; decay now slower
2nd review	3 days after 1st	Retention resets again; decay slower still
3rd review	7 days after 2nd	Material entering long-term memory
4th review	14 days after 3rd	Decay very slow; minimal effort to recall
5th review	30 days after 4th	Material effectively consolidated

Without spacing, a student re-reading the same notes for 3 hours straight gets far less retention than a student who spends 20 minutes on day 1, 10 minutes on day 2, and 10 minutes on day 4.

A concrete example: you learn a set of 20 flashcards on binary numbers on Monday.

Day	Action
Monday	Learn the 20 cards for the first time
Tuesday	Review all 20 cards (first spaced review)
Friday	Review cards you got wrong on Tuesday + spot-check the rest
Following Friday	Full review of all 20 cards
Two weeks later	Quick review — most cards retrieved easily

The total review time over five weeks is roughly 60–80 minutes. A student who instead re-reads the same notes for 60 minutes on Monday will retain far less two weeks later.

The key principle: time your reviews to coincide with the point when you're about to forget. Reviewing material you already know perfectly wastes time; reviewing material you've partially forgotten produces the strongest memory trace.