Spacing and Testing Effects: A Deeply Critical, Lengthy, and At Times Discursive Review of the Literature

What appears to be a simple pattern of results—distributed-study opportunities usually produce better memory than massed-study opportunities—turns out to be quite complicated. Many ‘‘impostor’’ effects such as rehearsal borrowing, strategy changes during study, recency effects, and item skipping complicate the interpretation of spacing experiments. We suggest some best practices for future experiments that diverge from the typical spacing experiments in the literature. Next, we outline the major theories that have been advanced to account for spacing studies while highlighting the critical experimental evidence that a theory of spacing must explain. We then propose a tentative verbal theory based on the SAM/REM model that utilizes contextual variability and study-phase retrieval to explain the major findings, as well as predict some novel results. Next, we outline the major phenomena supporting testing as superior to restudy on long-term retention tests, and review theories of the testing phenomenon, along with some possible boundary conditions. Finally, we suggest some ways that spacing and testing can be integrated into the classroom, and ask to what extent educators already capitalize on these phenomena. Along the way, we present several new experiments that shed light on various facets of the spacing and testing effects.