Distributed Practice Effects

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Learning and memory are generally improved by repetition. However, not all repetitions are equally beneficial. The effectiveness of repetitions depends in part on their temporal distribution. A piece of information studied on several occasions widely spaced apart in time will be remembered better than a similar fact studied on several occasions close in time.

The advantage of distributed repetitions over spaced repetitions has long been known. Hermann Ebbinghaus discussed distributed practice effects in his classic 1885 monograph on memory. He noted 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" (p. 89). Similarly, Jost (1897) formulated the advantage of distributed over massed repetitions as one of his laws of memory. In subsequent decades, distribution of repetition became an important manipulation in the study of learning and memory. Because many different procedures were used and many conflicting results were found, the overall pattern was long unclear, and investigators, such as Underwood (1961), sometimes despaired of being able to find consistent advantages for distributed practice over massed practice.

Among researchers on human memory, an important breakthrough was the research of Arthur Melton (1967), who used a procedure that became standard for many subsequent investigators. Participants saw a list of words presented one at a time. Some words were shown once on the list, while others were shown twice. Of the words that were shown twice, some were repeated in massed fashion; that is, they were presented twice in a row. Other words were repeated in spaced or distributed fashion; that is, they had their occurrences separated by one or more intervening words. After presentation of the list was complete, participants were asked to recall the items in any order. Melton found that the probability of recall for repeated items increased as a positive function of the number of intervening items between presentations. This advantage for distributed repetitions over massed repetitions is often called the spacing effect; the fact that memory for spaced items may improve somewhat as the distribution between repetitions is increased further is sometimes called the lag effect.

Are massed presentations ever more effective than distributed repetitions? Glenberg (1976) presented evidence that massed presentations may be superior if memory is only required for a very short interval. When the memory test is given almost immediately after the last presentation, massed repetitions may lead to superior memory than spaced repetitions. Evidently, the benefit one obtains from having multiple study episodes right before the test can sometimes be greater than the advantage usually gained from distributed rehearsal. Therefore, cramming may be a reasonably effective study strategy if one is sure that the test will occur as soon as the cramming is finished. However, distributed practice consistently results in better long-term retention.

Importance of Distributed-Practice Effects

One reason why the effects of distributed practice on memory are seen by many researchers as important is that they have wide generality. One can find these effects in many different subject populations, including nonhuman animals (Davis, 1970), human infants (Cornell, 1980), children (Toppino, 1991), and the elderly (Balota, Duchek, and Paullin, 1989; Benjamin and Craik, 2001). These effects are found on all sorts of memory tests and on a wide variety of different materials.

Distributed practice can also be used to improve learning and retention of meaningful material. For example, Rea and Modigliani (1985) showed that distributed practice facilitates memory for spelling and multiplication facts. Reder and Anderson (1982) found that distributed practice leads to improved memory for information from textbooks, relative to massed practice. Dempster (1987) showed that learning of the definitions of uncommon English words benefited from distributed practice. Bahrick and Phelps (1987) demonstrated that retention of Spanish vocabulary words over an eight-year period was greatly affected by distribution of practice. In short, distribution of practice seems to be as important in the mastery of classroom material as it is in the memorization of lists in a laboratory. Dempster (1988) called for educators to be more sensitive to the importance of this variable for classroom instruction, as distributing practice may improve retention without requiring additional time and resources.

Study-Phase Retrieval Accounts

Many different theories have been used to explain distributed-practice effects. Although the details of these theories vary, contemporary accounts can generally be grouped into three separate approaches. One such approach emphasizes the importance of study-phase retrieval (Braun and Rubin, 1998). This approach assumes that one way in which repetition helps to improve memory is by reminding the learner of earlier encounters with the studied information. For example, if a particular word occurs twice on a list, the second presentation may remind the learner of the first presentation. The learner may then think about the first presentation again, and this retrieval and added rehearsal would make the first presentation particularly memorable. To explain why the distribution of repetitions affects memory, one must add on the assumption that this reminding process benefits memory only if sufficient time has passed to guarantee that the first presentation is not already being consciously rehearsed.

It is reasonable that an encounter with a stimulus may serve as a reminder of earlier encounters. However, at least one critical claim of this approach is unsupported. A study-phase retrieval account predicts that memory for the first occurrence of a repeated item is affected by distribution of practice. Although it is difficult to distinguish between memory for the first occurrence and memory for the second occurrence of a repeated item, what little evidence researchers have suggests that it is memory for the second occurrence, not the first, that is affected by distribution of practice (Hintzman, Block, and Summers, 1973). Although a study-phase retrieval process may contribute to effects of distributed practice, there is little direct evidence in favor of this approach.

Deficient-Processing Accounts

An alternative approach to explaining distributed-practice effects is to claim that learners do not process the second occurrence of an item as fully when it is repeated in massed fashion as when it is repeated in spaced fashion. That is, the second occurrence of a massed repetition is not given as much attention or rehearsal as the second occurrence of a spaced repetition.

Zechmeister and Shaughnessy (1980) suggested that deficient processing of massed items may reflect rehearsal strategies on the part of learners. For example, in the case of people memorizing a list of words, the participants do not just rehearse each item as it is presented; rather, they presumably divide their rehearsal between the current item and previous items. The amount of rehearsal that a person devotes to an item presumably depends on how difficult the item appears to be. Zechmeister and Shaughnessy found that participants overestimate the extent to which they would remember massed repetitions. Because people may mistakenly believe that massed items will be easy to remember, they may not devote as much rehearsal and attention to them as they do to spaced items. Indeed, when participants are asked to rehearse aloud or to pace themselves through a list, massed repetitions seem to receive less rehearsal than distributed repetitions.

Another version of this approach (advocated by Hintzman, 1974) attributes distributed-practice effects to deficient processing of the second occurrence of massed repetitions but views this as a result of an automatic, unconscious process, not as the result of a deliberate strategy. This type of account has the advantage of being applicable to subject populations, such as human infants, that show distributed-practice effects but that presumably do not employ conscious rehearsal strategies.

Encoding-Variability Accounts

Encoding-variability accounts of distributed-processing effects assume that spacing between repetitions facilitates memory by increasing the likelihood that each occurrence of a repeated item is stored in a very different way in memory. This sort of approach may best be explained by way of an analogy. Imagine that a business office is very disorganized, and that workers often have difficulty finding papers they need. If they have an important document that they want to be able to find at a later time, it would make sense for them to make multiple copies of it. However, it would be wise for them to keep each of the copies in different places and filed in different ways; this would increase the probability that they would find at least one copy when they need it. Encoding-variability accounts assume that distributed practice increases the probability that different occurrences of a repeated item will be stored in memory in different ways, thereby increasing the probability that a person would be able to retrieve at least one occurrence.

Different theorists have approached encoding variability in different ways. Landauer (1976) argued that distribution of practice directly influenced the location of memories of specific occurrences. Other theorists, such as Gartman and Johnson (1972) and Glenberg (1979), have emphasized that distribution of practice may increase the probability that a repeated item will be interpreted or analyzed differently at each occurrence. What these approaches have in common is the assumption that repeated items are remembered better if they are studied differently at each occurrence and that distributed rehearsal increases the probability that variability in study will occur.

Multiprocess Accounts

It seems increasingly likely that there will be no single explanation for why distributed practice improves memory. Rather, there are several reasons why distributed repetitions are more effective than massed repetitions, and all three of the major approaches (study-phase retrieval, deficient processing, encoding variability) may apply under some circumstances. Greene (1989) and Russo, Parkin, Taylor, and Wilks (1998) have offered multiprocess accounts that combine these approaches. The nature of the subject population, the stimulus material, and the experimental procedure all seem to determine the nature of the processes that underlie the advantage for distributed practice.

Although the theoretical explanation for the effects of distributed practice is likely to be complex, there is no question that these effects are powerful and of wide generality. The fact that distributed practice leads to superior retention than massed practice needs to be taken into account by both experimenters and educators.



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Arthur M.Glenberg

Revised byRobert L.Greene

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Distributed Practice Effects

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