Hermann Ebbinghaus, answered from the texts and cited to the page.
Repetitions distributed across time accomplish more than the same number massed into a single sitting. That is the finding, stated plainly. When I learned nine 12-syllable series by repeating them across several successive days, the number of repetitions required fell steadily: 158 on the first day, then 109, then 75, then 56 — each day's relearning costing less than the last.1
The comparison that matters most is this: 68 repetitions made on a single day were needed to fix a 12-syllable series well enough for errorless recital the next morning, whereas 38 repetitions distributed across the three preceding days produced the same result.2 The difference is large enough that I felt confident calling it significant, and it makes probable the general principle that with any considerable number of repetitions a suitable distribution of them over a space of time is decidedly more advantageous than the massing of them at a single time.3
The effect of repetitions is not constant. Early repetitions purchase a great deal; later ones, as the series becomes more firmly fixed, purchase less and less.4 This means that cramming — returning again and again to the same material in one sitting — runs into diminishing returns precisely when the material feels most secure. Spreading the work lets the trace consolidate, so that each new session finds it somewhat settled and builds upon it more efficiently.
The schoolboy who reviews his vocabulary in the morning what he half-learned the night before is, without knowing it, obeying this law.5 The teacher who reserves part of a lesson period for later review is doing the same. The method that practice naturally hits upon turns out, when measured, to be the method the numbers recommend.
We found, for example, that nine 12-syllable series were learned on six successive days by means of 158, 109, 75, 56, 37 and 31 repetitions.Memory A Contribution to Experimental Psychology
For the relearning of a 12-syllable series at a definite time, accordingly, 38 repetitions, distributed in a certain way over the three preceding days, had just as favorable an effect as 68 repetitions made on the day just previous.Memory A Contribution to Experimental Psychology
It makes the assumption probable that with any considerable number of repetitions a suitable distribution of them over a space of time is decidedly more advantageous than the massing of them at a single time.Memory A Contribution to Experimental Psychology
The effect of the repetitions is at first approximately constant... Gradually the effect becomes less; and finally, when the series has become so firmly fixed that it can be repeated almost spontaneously after 24 hours, the effect is shown to be decidedly less.Memory A Contribution to Experimental Psychology
The schoolboy doesn't force himself to learn his vocabularies and rules altogether at night, but knows that he must impress them again in the morning. A teacher distributes his class lesson not indifferently over the period at his disposal but reserves in advance a part of it for one or more reviews.Memory A Contribution to Experimental Psychology