The spacing effect was known as early as 1885 when Ebbinghaus published the results of his seminal experimental work on memory. With himself as the subject, Ebbinghaus found that for a single 12-syllable series, 68 immediately successive repetitions had the effect of making possible an errorless recital after seven additional repetitions on the following day. However, the same effect was achieved by only 38 distributed repetitions spread over three days. On the basis of this and other related findings, Ebbinghaus concluded 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" (Ebbinghaus, 1885/1913, p. 89).

In this article, I have considered nine possible impediments to the implementation of research findings in the classroom. Of the nine, five appear to apply to the spacing effect. These include the ahistorical character of research on the spacing effect, some failures to obtain the effect with school-like activities, a paucity of impressive classroom demonstrations of the phenomenon, limited knowledge of classroom practice, and an incomplete understanding of the psychological bases of the spacing effect.