Education researchers usually now talk of increasing interval retrieval practice whereas spaced repetition system, which is favoured by computer programmers.
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He called the graph below the die Vergessenskurve1 or in English the forgetting curve, or since he was German, die Vergessenskurve1.
It shows that as time goes on, memory fades.
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1 see a nice graph in https://de.wikipedia.org/wiki/Vergessenskurve Über das Gedächtnis. Untersuchungen zur experimentellen Psychologie. Duncker & Humblot, Leipzig 1885 and https://en.wikipedia.org/wiki/Forgetting_curve to read about it in English.
Video on how the spaced repetition system works
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