Why Retrieval and Spacing Work

Testing is not only measuring

In 2006 Henry Roediger and Jeffrey Karpicke reported two experiments in Psychological Science. Students read prose passages and then either took one or three immediate free recall tests without feedback, or restudied the material the same number of times. Retention was checked after 5 minutes, 2 days or 1 week. The question was simple. Does taking a test help memory, or does it only measure what memory already holds? Testing and restudying were repeated the same number of times, so the groups differed in the kind of activity and not in how often it occurred.

What they found

Restudying produced better recall on the 5 minute test. On the delayed tests, prior testing led to substantially better retention than restudying. Restudying also increased students' confidence in their memory. The authors concluded that "Testing is a powerful means of improving learning, not just assessing it." We think the confidence finding matters most. Rereading feels productive, and the feeling is not a reliable guide to what will be remembered.

For a learner, this suggests a plain exercise: close the book, write down what you remember, check it against the text, and repeat after a pause. We offer that as our own reading, not as a finding of either paper.

Spacing and the length of the gap

In the same year, Nicholas Cepeda and colleagues published a quantitative synthesis of distributed practice in Psychological Bulletin. It covered 839 assessments from 317 experiments in 184 articles. The review compared massed presentations, where study episodes come back to back, with spaced episodes, and it also examined the lag between episodes and expanding intervals. The authors found that the gap between study sessions and the length of time until the final test work together. The best gap between sessions grows as the time until the test grows. We think a single rule such as "review every day" is unlikely to fit every goal.

What we do not know

Both papers report experiments, and the synthesis concerns verbal recall tasks. Neither tells us how long the gap should be for a particular course, and we have not tested it ourselves. We do not know how far the results carry to every subject, skill or learner, and we would not claim that they do. Our reading is narrower, and it is opinion. We think a course is hard for the right reasons when it asks learners to retrieve what they have studied and schedules the return to it, rather than offering one more reading. If you read the papers and conclude otherwise, we would like to hear why.