Decomposing the interaction between retention interval and study/test practice: the role of retrievability.

Even without feedback, test practice enhances delayed performance compared to study practice, but the size of the effect is variable across studies. We investigated the benefit of testing, separating initially retrievable items from initially nonretrievable items. In two experiments, an initial test determined item retrievability. Retrievable or nonretrievable items were subsequently presented for repeated study or test practice. Collapsing across items, in Experiment 1, we obtained the typical cross-over interaction between retention interval and practice type. For retrievable items, however, the cross-over interaction was quantitatively different, with a small study benefit for an immediate test and a larger testing benefit after a delay. For nonretrievable items, there was a large study benefit for an immediate test, but one week later there was no difference between the study and test practice conditions. In Experiment 2, initially nonretrievable items were given additional study followed by either an immediate test or even more additional study, and one week later performance did not differ between the two conditions. These results indicate that the effect size of study/test practice is due to the relative contribution of retrievable and nonretrievable items.