What Is Interleaving?

Interleaving means mixing different kinds of material in one study session rather than practicing one kind at a time. Instead of ten problems of type A, then ten of type B, you shuffle them. Learners usually perform worse during interleaved practice but better on later tests, especially when the task is to tell similar things apart.

Where the research comes from

Interleaving grew out of motor-learning research in the 1970s and 1980s, where varied practice schedules beat repetitive drills on later tests. Two well-known cognitive studies made it popular:

• Kornell and Bjork (2008) asked students to learn the styles of different landscape painters from examples. Students who saw paintings by different artists mixed together were better at identifying the artist of new paintings than students who saw each artist's paintings in a block. Most students nonetheless believed blocking had worked better.
• Rohrer and Taylor (2007) found that students who practiced mixed geometry problems (volumes of different solids) did much better on a delayed test than students who practiced each problem type in a block.

The main explanation is discrimination: when items are mixed, you must work out which category or strategy applies, and you notice the differences between them. Blocking hides that decision.

Interleaving vs blocking vs spacing

Interleaving and spacing often occur together, because mixing items puts gaps between repetitions of each one. But they are distinct: you could space reviews of a single topic across days without ever mixing it with another.

A vocabulary example

Why it matters for learning words

Advanced vocabulary is largely about distinctions: economic vs economical, continual vs continuous, persuade vs convince. Learning these one at a time often leaves you able to define each but unable to choose correctly in a sentence. Mixed practice targets the choice itself.

There is also a caution. Research on teaching semantically related words together (for example all fruit names in one lesson) suggests that presenting brand-new, tightly related words at the same time can increase confusion for beginners. A practical rule: learn new words first, then interleave them in review once each has a foothold.

How to apply it

• Shuffle review decks instead of studying them by topic or by the day you added them.
• Build "confusable sets" of near-synonyms and practice choosing between them in context.
• Mix task types: recall a meaning, fill a cloze gap, write a sentence.
• Reading widely is naturally interleaved; words you saved from one site show up mixed with others elsewhere. Wordie's highlighting of saved words across the sites you visit gives this kind of varied exposure.

Common misconceptions

• "Interleaving means random multitasking." It means mixing related items within one task, not switching between unrelated activities.
• "It always helps." Benefits are clearest for similar, confusable categories.
• "If it feels harder, it's worse." Interleaving is a classic desirable difficulty.

Sources: Kornell, N., & Bjork, R. A. (2008). Learning concepts and categories: Is spacing the "enemy of induction"? Psychological Science. Rohrer, D., & Taylor, K. (2007). The shuffling of mathematics problems improves learning. Instructional Science.