What Ebbinghaus did
In the early 1880s Hermann Ebbinghaus used himself as his only subject. He memorised lists of meaningless syllables, chosen so that existing knowledge would not help, and tested how much he retained after intervals from twenty minutes to a month.
He measured memory by "savings": how much less time it took to relearn a list than to learn it the first time. He published the results in 1885, and they became one of the first quantitative findings in psychology.
The shape of the curve
The key finding was not a single number but a shape. Forgetting is steep at first and then slows down: a large share of what is lost is lost early, and what survives the first days fades far more gradually after that.
Exact percentages depend heavily on the material, how well it was learned and how it is tested, so figures quoted online as universal rules should be treated with caution. Meaningful material you understand fades much more slowly than nonsense syllables. The shape, though, holds up: a 2015 replication following Ebbinghaus's method found a very similar curve.
What flattens it
| Factor | Effect |
|---|---|
| Reviewing at spaced intervals | Each review resets the curve and makes the next decline slower |
| Recalling rather than rereading | Active retrieval strengthens memory more than passive exposure |
| Understanding and meaning | Material connected to what you already know fades more slowly |
| Sleep | Consolidates what was learned during the day |
Why it matters for learning
If most forgetting happens early, the most valuable review is the first one, and it should come soon. Each later review can come after a longer gap. That is exactly the schedule spaced repetition follows: short gaps at the start, widening as the memory becomes more stable.
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