Spacing effects in learning: A temporal ridgeline of optimal retention

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