The forgetting curve: what Ebbinghaus actually found
You have probably seen the curve that drops off a cliff in the first day. The original data are real, and a modern replication confirms their shape. But the famous numbers attached to them are mostly wrong, and the curve says less about your lectures than it seems
Nila team · Updated · 3 min read
Where the curve comes from
In the 1880s Hermann Ebbinghaus memorized lists of nonsense syllables, such as DAX or BOK, and measured how quickly he forgot them. He was his own only subject. He did not measure how much he could recall; he measured savings, how much less time it took to relearn a list than it had taken to learn it the first time.
In 2015 Murre and Dros repeated the experiment as closely as they could, again with a single subject: Dros himself [1]. The shape held: a steep drop in the first hour, then a slow tail. Here are the savings both of them recorded:
| Time since learning | Ebbinghaus (1880s) | Replication (2015) |
|---|---|---|
| 20 minutes | 58% | 47% |
| 1 hour | 44% | 37% |
| 9 hours | 36% | 28% |
| 1 day | 34% | 32% |
| 2 days | 28% | 23% |
| 6 days | 25% | 17% |
| 31 days | 21% | 4% |
The replication also shows a small bump upward at 24 hours. Some researchers attribute it to sleep; the study did not test that.
The numbers people quote
You will often read that we forget “70% of what we learn within 24 hours” or “90% within a week.” Neither is what the data say.
- “70% in a day” most likely comes from subtracting Ebbinghaus’s 34% savings after one day from 100. But savings are not recall. 34% savings means relearning took a third less time, not that two thirds of the material was gone.
- “90% in a week” does not appear in either dataset.
- The material was meaningless syllables, chosen precisely because they connect to nothing. Lectures are the opposite: ideas that connect to each other and to what you already know. A curve for nonsense syllables is a poor guide to how fast a lecture you understood will fade.
The replication’s authors also note that savings, as a measure, has largely been abandoned by researchers as unreliable [1].
What actually slows forgetting
The useful lesson of the curve is not the exact numbers but the shape: forgetting is fastest soon after learning, and each well-timed review flattens what comes next. Three things reliably change the curve.
Spaced review. The same number of reviews, spread out, keeps material much longer than reviews packed together. In a study of 1,354 people, the best gap between two sessions improved recall by 64% relative to no gap [2]. See spaced repetition.
Recall instead of rereading. In Roediger and Karpicke’s experiment, students who reread a passage lost about half of what they had recalled after a week; students who practiced recalling it lost about 14% [3]. See retrieval practice.
Sleep between sessions. Mazza and colleagues had people learn and then relearn word pairs 12 hours later, either across a night of sleep or across a day awake [4]. The sleep group needed about half as many rounds to relearn (3 against 6), and six months later remembered about two and a half times as much. A group that slept but relearned only once did no better than the wake group: the benefit came from sleep and relearning together. The study was small (40 people) and time of day was not fully separated from sleep.
What this means for a lecture
- Expect to lose a lot quickly. That is normal, not a sign you studied badly.
- Make the first review a day later, after a night’s sleep, and make it a recall rather than a reread.
- Keep coming back at longer gaps. Each successful recall buys more time before the next one is needed.
- Learn the ideas as connected ideas. Material you understand and can link to other things is not a list of nonsense syllables.
References
- Murre, J. M. J., & Dros, J. (2015). Replication and analysis of Ebbinghaus’ forgetting curve. PLOS ONE, 10(7), e0120644. doi.org/10.1371/journal.pone.0120644
- Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095–1102. doi.org/10.1111/j.1467-9280.2008.02209.x
- Roediger, H. L., & Karpicke, J. D. (2006). Test-enhanced learning: Taking memory tests improves long-term retention. Psychological Science, 17(3), 249–255. doi.org/10.1111/j.1467-9280.2006.01693.x
- Mazza, S., Gerbier, E., Gustin, M.-P., Kasikci, Z., Koenig, O., Toppino, T. C., & Magnin, M. (2016). Relearn faster and retain longer: Along with practice, sleep makes perfect. Psychological Science, 27(10), 1321–1330. doi.org/10.1177/0956797616659930