Because people forget much of what they learn, students could benefit from learning strategies that provide long-lasting knowledge. Yet surprisingly little is known about how long-term retention is most efficiently achieved. Here we examine how retention is affected by two variables: the duration of a study session and the temporal distribution of study time across multiple sessions. Our results suggest that a single session devoted to the study of some material should continue long enough to ensure that mastery is achieved but that immediate further study of the same material is an inefficient use of time. Our data also show that the benefit of distributing a fixed amount of study time across two study sessions – the spacing effect – depends jointly on the interval between study sessions and the interval between study and test. We discuss the practical implications of both findings, especially in regard to mathematics learning.

Our research on learning enhancement has been focusing on the consequences for learning and forgetting of some of the more obvious and concrete choices that arise in instruction, including questions such as these: How does spacing of practice affect retention of information over significant retention intervals(up to 1 year)? Do spacing effects generalize beyond recall of verbal materials? Is feedback needed to promote learning, and must it be immediate? Although retrieval practice has been found to enhance learning in comparison with additional study, does it actually reduce the rate of forgetting? Can retrieval practice effects be extended to nonverbal materials? We suggest that as we begin to find answers to these questions, it should become possible for cognitive psychology to offer nonobvious advice that can be applied in a variety of instructional contexts to facilitate learning and reduce forgetting.

More than a century of research shows that increasing the gap between study episodes using the same material can enhance retention, yet little is known about how this so-called distributed practice effect unfolds over nontrivial periods. In two three-session laboratory studies, we examined the effects of gap on retention of foreign vocabulary, facts, and names of visual objects, with test delays up to 6 months. An optimal gap improved final recall by up to 150%. Both studies demonstrated nonmonotonic gap effects: Increases in gap caused test accuracy to initially sharply increase and then gradually decline. These results provide new constraints on theories of spacing and confirm the importance of cumulative reviews to promote retention over meaningful time periods.

More than a century of research shows that increasing the gap between study episodes using the same material can enhance retention, yet little is known about how this so-called distributed practice effect unfolds over nontrivial periods. In two three-session laboratory studies, we examined the effects of gap on retention of foreign vocabulary, facts, and names of visual objects, with test delays up to 6 months. An optimal gap improved final recall by up to 150%. Both studies demonstrated non-monotonic gap effects: Increases in gap caused test accuracy to initially sharply increase and then gradually decline. These results provide new constraints on theories of spacing and confirm the importance of cumulative reviews to promote retention over meaningful time periods.

Once material has been learned to a criterion of one perfect trial, further study within the same session constitutes overlearning. Although overlearning is a popular learning strategy, its effect on long-term retention is unclear. In two experiments presented here, 218 college students learned geography facts (Experiment 1) or word definitions (Experiment 2). The degree of learning was manipulated and measured via multiple test-with-feedback trials, and participants returned for a final cued recall test between one and nine weeks later. The overlearners recalled far more than the low learners at the one-week test, but this difference decreased dramatically thereafter. These data suggest that overlearning (and its concomitant demand for additional study time) is an inefficient strategy for learning material for meaningfully long periods of time.

To achieve enduring retention, people must usually study information on multiple occasions. How does the timing of study events affect retention? Prior research has examined this issue only in a spotty fashion, usually with very short time intervals. To characterize spacing effects over significant durations, over 1350 individuals were taught a set of facts and – after a gap of up to 3.5 months – given a review on the same facts. A final test was administered at a further delay of up to 1 year. At any given retention interval, an increase in the inter-study gap at first increased, and then gradually reduced, test performance. The optimum gap value was about 20% of the test delay for delays of a few weeks, falling to about 5% when delay was one year. The interaction of gap and test delay implies that many educational practices are likely to be highly inefficient.