For over a century cognitive psychologists have been studying spaced practice and retrieval practice, resulting in a great deal of evidence that these two strategies work very well to improve student learning. However, no single experiment or paper is going to be able to answer, in full, the question “how does this strategy work in education?”. When we point to a single experiment, or even a set of experiments, it can be quite easy to point out the ways in which the experiment(s) may not be a perfect test of the phenomenon, or might not measure everything we want to measure. Each experiment or set of experiments has strengths and limitations, but by conducting the research in a lot of different ways, we gain more confidence in the recommendations and better understand the way these processes work. (Of course, we still have plenty of work to do as not every nuanced question has been answered.)
