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Suppose you are working through three chapters, three painters, or three kinds of math problems. Almost everyone schedules the work the same way: finish topic A, then move to topic B, then C. Psychologists call this blocked practice, and it has an obvious logic. You stay in one groove until the groove feels smooth. The alternative, mixing the topics together in an A-B-C-B-A-C sort of order, is called interleaving, and it feels scattered and inefficient. The research record says the scattered-feeling schedule is the one that wins, usually by a wide margin, on any test that arrives later than the practice itself.
The cleanest demonstrations come from mathematics, where Doug Rohrer and Kelli Taylor spent years comparing the two schedules. In their 2007 study, The Shuffling of Mathematics Practice Problems Boosts Learning (opens in new tab), college students learned to find the volumes of several obscure solids, practicing with problems either blocked by type or randomly mixed. On a test one week later, the mixed group's performance was vastly superior, an effect later work pegged at a Cohen's d of 1.34, which is enormous by the standards of education research.
Their 2010 follow-up with fourth graders (opens in new tab) added the detail that makes interleaving so counterintuitive. During the practice session itself, the interleaved group did worse than the blocked group. One day later, the interleaved group's test scores were roughly double. The schedule that looked like it was failing while it happened produced twice the learning.
The effect holds up outside the lab. A 2015 classroom study (opens in new tab) by Rohrer, Dedrick, and Stershic ran the comparison with seventh graders over three months of regular lessons. Tested one day after review, interleaved practice beat blocked practice 80 percent to 64 percent. Tested thirty days after, the gap widened to 74 percent versus 42 percent. The authors point out something remarkable in those numbers: a thirtyfold increase in test delay cost the interleaved group less than a tenth of its score. Blocked learning evaporated; interleaved learning barely moved.
Interleaving is not just for procedures. Nate Kornell and Robert Bjork asked whether it also helps inductive learning, the kind readers do constantly: absorbing a writer's style, a school of thought, a category of argument, from examples. In their 2008 experiments (opens in new tab), participants studied landscape paintings by twelve unfamiliar artists, with each artist's works shown either consecutively or interleaved with the others. Then they had to attribute new, unseen paintings to the right artist. Interleaved study produced 74 percent accuracy; blocked study produced 55 percent, which was not reliably better than chance.
The sting is in the metacognition. In the first experiment, 78 percent of participants performed better in the interleaved condition, and 78 percent judged that blocking had worked as well or better. People walked out of the study having just demonstrated the superiority of interleaving on their own test, and attributed their success to the schedule that failed them. Blocking produces the same trap as re-reading: a feeling of fluency during study that has almost nothing to do with what survives until tomorrow. It is the same illusion that makes re-reading feel productive while doing very little.
Two mechanisms carry most of the effect. The first is discriminative contrast. Follow-up work by Birnbaum, Kornell, and the Bjorks (opens in new tab) found that interleaving helps because it puts members of different categories side by side, which spotlights the differences between them; Kang and Pashler showed that adding mere time gaps between blocked examples, without the juxtaposition, did not help. Blocked study lets you see what one artist's paintings share. Interleaved study forces you to see what separates this artist from that one, and telling things apart is usually the skill the test demands.
The second mechanism is choice. When a practice set contains only one kind of problem, you never have to ask the first and hardest question: what kind of problem is this? The procedure is given; you just execute it. Mixed practice makes you select the strategy before applying it, which is exactly what exams, and life, require. Interleaving also delivers spacing for free, since mixing topics guarantees that any two encounters with the same topic are separated by others, and distributed practice is one of the best-supported techniques in the literature. Harder in the moment, stronger later: interleaving is a textbook desirable difficulty.
Most interleaving research uses problem sets and category examples, but the principle transfers to a reading life with little friction.
Blocked practice survives because it feels like progress and fits how books, courses, and to-do lists are organized. Interleaving wins because it rehearses the thing that mastery actually consists of: telling ideas apart and knowing which one to reach for. Arrange your reading so the mixing happens, and let the discomfort reassure you.
