CONCEPT
Interleaving Effect
The finding that mixing different types of problems during practice—rather than completing all problems of one type before moving to the next—impairs immediate performance but enhances long-term retention, transfer, and the ability to discriminate between problem categories.
Interleaving, the practice of mixing different problem types or topics within a single study session, produces a cognitive benefit that blocked practice (studying one type at a time) cannot replicate: it forces discrimination. In blocked practice, the problem type is given by context—all problems in this section are quadratic equations, so apply the quadratic formula. The learner never has to determine what kind of problem she faces. In interleaved practice, each new problem could be any of several types, forcing the learner to identify the category before selecting a strategy. This additional categorization step—determining which approach applies to which problem—builds the flexible diagnostic skill that defines competent performance outside the practice environment. Studies of mathematics learning, sports training, and category acquisition consistently show that interleaved practice produces worse performance during training (more errors, slower completion) and better performance on delayed tests, especially when those tests require transfer to novel contexts. The effect is a textbook instance of the
performance-learning dissociation