The spacing effect is a desirable difficulty for learning (along with the testing effect) that helps produce long lasting, durable memory traces, but has also been ignored in education. The spacing effect is when the learning of material takes place over long periods of time. Usually, when we teach something, we concentrate the presentation of related information in a short amount of time. Once that information is learned, we move on to different information. Longer lasting and more durable memory traces result from the spacing of the learning of information in time.

In an illustration of this effect, in a study by Bahrick (1979), subjects were learning Spanish words. Their lessons were spaced out as occurring once-a-day, once-a-week, or once-a-month. Table 3 shows you how well the subjects remembered the words from previous lessons during the review that was administered before each successive lesson.

Table 3 (based on Bahrick 1979)

Mean percentage of correct recall of Spanish words on successive learning sessions.

                                               Study Period

Lesson Interval                    2          3          4          5          6

Once-a-Day                          53%      86%      94%      96%      98%

Once-a-Week                       39%      68%      83%      89%      94%

Once-a-Month                     21%      51%      72%      79%      82%

Clearly, across the sessions, having lessons once-a-day, for six days, produces better recall than having lessons once-a-month, for six months. However, when the subjects were brought back for a test of their Spanish words 30 days after their final learning session had finished, the once-a-day group scored 68%, the once-a-week group scored 86%, and the once-a-month group scored 95% (higher than the test results immediately following the last session). Learning over a longer period of time requires more work to remember what you learned before, and when more work is involved in the learning the memory traces are strengthened, making what is learned far more durable.

Once again we see a simple change in the way we do things having a dramatic effect on remembering information. I am unaware of any learning program that uses the spacing effect at all to help students remember information. Of course, the findings are very recent and that might have something to do with them not being adopted by the educational community. They were first observed and reported over a century and a quarter ago.