What the forgetting curve is
It is a graph. Time since you learned something runs along the bottom. How much of it you still have runs up the side. The line drops steeply at first and then levels out.
The shape matters more than any one number. Most forgetting happens soon after learning. What survives the first day fades slowly after that.
What Ebbinghaus measured
Hermann Ebbinghaus, a German psychologist, ran the experiment on himself and published it in 1885. He learned lists of nonsense syllables, such as “DAX” and “BOK”, until he could say a list without a mistake. Then he waited, and learned the same list again.
His measure was savings: how much less time the second learning took. If relearning takes half the time, the saving is 50 percent. These are his results:
| Time since learning | Savings on relearning |
|---|---|
| 20 minutes | 58% |
| 1 hour | 44% |
| 9 hours | 36% |
| 1 day | 34% |
| 2 days | 28% |
| 6 days | 25% |
| 31 days | 21% |
More than half of the saving is gone in the first hour. Between day 6 and day 31, almost nothing more is lost.
Does it hold up?
Yes. In 2015, Murre and Dros repeated the experiment with the same method: one person, nonsense syllables, about 70 hours of learning and relearning. Their curve was close to the one from 1885.
Two cautions. First, the numbers are for nonsense syllables, which are hard to remember on purpose. Material that means something to you fades more slowly. Second, the curve is an average shape, not a law. Sleep, interest and what you already know all move it.
What flattens the curve
Two things, and both are well tested.
- Spacing. A review restarts the curve, and the new curve is flatter than the last one. Each review can come later than the one before. The best moment is just before you would forget.
- Retrieval. The review must be a question, not a second reading. In 2006, Roediger and Karpicke had students either reread a text or try to recall it. Rereading won on a test five minutes later. Recall won on tests two days and one week later.
The second finding explains a common trap. Rereading feels good and works for the next five minutes. Recall feels harder and works for the next week.
How Axo uses it
Every idea you learn in Axo gets a memory score and a next-review date. Keep Sharp asks you about the ideas that are due today, mixed across all your Paths. Answer well and the next review moves further out. Miss it and it comes back sooner.
Every review is a question. On review you type the answer from memory, and the question is asked another way than you first met it. The chart on this page is a model of that schedule, not a measurement of you.