" So far, we've focused on retrieval practice – improving learning by bringing information to mind. There are many ways to define it and many ways to foster retrieval in our classrooms. But when and how often? Is more retrieval practice better than less? Here's the key to unleashing retrieval practice: Don't have students simply engage in retrieval once or even repeatedly. In this chapter, we feature research and strategies for two more powerful teaching approaches: spacing and interleaving. "

― Pooja K. Agarwal , Powerful Teaching: Unleash the Science of Learning