Sayings such as "Practice makes perfect" illustrate the well-known fact that repetition improves learning. This was discussed by numerous ancient and medieval thinkers and was demonstrated empirically by Hermann Ebbinghaus, the first researcher to carry out a prolonged series of experiments on human memory. In a classic 1885 book, Ebbinghaus showed that retention of information improves as a function of the number of times the information has been studied. Since the time of Ebbinghaus, countless investigators have used repetition to study learning and memory.

Although experimenters typically find a consistent relationship between repetition and learning, numerous authors (Guthrie, 1935) have pointed out that this does not necessarily mean that the learning process itself has to be either gradual or continuous. Most learning situations contain a number of smaller facets or subproblems that must be mastered before learning is complete. It is possible that each of these...