A Grain of Truth in the Grain Size Effect: Retrieval Practice Is More Effective When Interspersed During Learning

Retrieval practice is a powerful method for consolidating long-term learning. When learning takes place over an extended period, how should tests be scheduled to obtain the maximal benefit? In an end-test schedule, all material is studied prior to a large practice test on all studied material, whereas in an interim test schedule, learning is divided into multiple study/test cycles in which each test is smaller and only assesses material from the preceding study block. Past investigations have generally found a difference between these schedules during practice but not during a final assessment, although they may have been underpowered. Five experiments confirmed that final assessment performance was better in students taught using interim than end tests in list (Experiments 1, 2, and 5) and paired associate (Experiments 3 and 4) learning, with a meta-analysis of all available studies (k = 19) yielding a small- to medium-sized effect, g = 0.25, 95% confidence interval [0.09, 0.42]. Experiment 5 finds that the higher level of practice retrieval success in interim tests contributes to the grain size effect, but the effect is eliminated if these tests are too easy. Additional analyses also suggest that the forward testing effect, in which tests promote subsequent learning, may be a major cause of the grain size effect. The practical and theoretical implications of these demonstrations of robust grain size effects are discussed.