Hermann Ebbinghaus, answered from the texts and cited to the page.
The finding is simple to state and striking to measure: repetitions spread across several days accomplish far more than the same number of repetitions crowded into one sitting.1 I came to this by a direct comparison. When I learned a set of series with 68 repetitions massed on a single day, I could reproduce them the following day after 7 further repetitions.
But when an equivalent effect was achieved by distributing 38 repetitions across three preceding days, the result on the fourth day was the same.1 Thirty-eight distributed repetitions did the work of sixty-eight massed ones. Even granting the uncertainty that attaches to numbers drawn from so few trials, the difference is large enough to be significant.
The underlying regularity is this: each repetition, within certain limits, adds to the depth of impression in approximately equal measure, and that depth is what I measured — not by what could be consciously recited, but by the savings in relearning, the work spared because a trace remained.2 What distributed practice appears to do is allow each day's repetitions to consolidate before the next day's are added, so that the later repetitions find a firmer ground and fix the series more economically.3
Practice itself had already discovered what the numbers confirm. The schoolboy does not cram his vocabularies entirely the night before; he knows he must go over them again in the morning. A teacher reserves part of the lesson period for review rather than spending it all at once.1 The method naturally employed in practice agrees with the result found in the laboratory — which is, I think, a fair sign that the laboratory has measured something real.
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
This readiness I measure by the amount of work saved in the relearning of any series as compared with the work necessary for memorising a similar but entirely new series.Memory A Contribution to Experimental Psychology
In this case the repetitions were distributed over several days and the attainment of the first possible reproduction was employed for their apportionment on the separate days.Memory A Contribution to Experimental Psychology