What is Interleaving?
Interleaving is a practice schedule that mixes different topics or problem types within a session instead of finishing one before starting the next. Mixing forces you to discriminate between problem types and choose the right approach, which hurts practice performance but improves later test performance, especially in math and category learning.
The contrast is with blocked practice, the default in most textbooks, where all problems of one kind appear together. Rohrer and Taylor had students practice mathematics problems either blocked by type or shuffled together; the shuffled group performed worse during practice but substantially better on a test a week later. Mixing denies you the shortcut of knowing in advance which method each problem needs.
That discrimination demand appears to be the active ingredient. Interleaving works best when the categories being mixed are easy to confuse, because the learner must repeatedly identify which kind of problem or example is in front of them, a skill blocked practice never trains. It also builds in spacing automatically, since each topic returns after a gap. The Dunlosky review rated interleaved practice moderate utility, with the strongest evidence in math.
For self-directed learners, interleaving means resisting the urge to exhaust one subject per session. Rotating between two or three related topics in a reading session, or mixing old and new material during review, trades comfortable momentum for durable discrimination.