Optimizing retrieval as a learning event: when and why expanding retrieval practice enhances long-term retention.

Retrieving information from memory makes that information more recallable in the future than it otherwise would have been. Optimizing retrieval practice has been assumed, on the basis of evidence and arguments tracing back to Landauer and Bjork (1978), to require an expanding-interval schedule of successive retrievals, but recent findings suggest that expanding retrieval practice may be inferior to uniform-interval retrieval practice when memory is tested after a long retention interval. We report three experiments in which participants read educational passages and were then repeatedly tested, without feedback, after an expanding or uniform sequence of intervals. On a test 1 week later, recall was enhanced by the expanding schedule, but only when the task between successive retrievals was highly interfering with memory for the passage. These results suggest that the extent to which learners benefit from expanding retrieval practice depends on the degree to which the to-be-learned information is vulnerable to forgetting.