Interleaving: why mixing topics helps you learn

What interleaving is

Blocked practice works through one type at a time: AAA, BBB, CCC. Interleaved practice mixes them: ABC, CAB, BCA. In a maths course that might mean shuffling problems from the last four chapters instead of doing one chapter's problems in a row. In pharmacology it means reviewing several drug classes together, in law several doctrines, in a language several tenses.

Interleaving isn't the same as spacing, though the two travel together. Spacing is about when you come back to a topic: after a gap, not straight away. Interleaving is about what sits next to it. Mixing topics in a session spaces each of them out automatically, which is one reason it's hard to separate the two. See the forgetting curve for why gaps help.

What the research found

Maths problems

In a 2007 study, Doug Rohrer and Kelli Taylor taught college students to find the volumes of four unusual geometric solids. One group practised the problems in blocks, one kind at a time. The other practised the same problems shuffled. During practice the blocked group did better. On a test a week later, the interleaved group did far better.

The same pattern has turned up with schoolchildren. Taylor and Rohrer found it in 2010 with fourth-graders solving problems about prisms. In a 2015 classroom study, Rohrer, Robert Dedrick and Sandra Stershic gave seventh-grade classes either blocked or interleaved homework over several months. The interleaved students scored higher on a later test, and the advantage was bigger when that test came a month later rather than the next day.

Paintings

In 2008, Nate Kornell and Robert Bjork had people study paintings by twelve artists, either all of one artist's paintings in a row or all of them mixed. Then they showed new paintings and asked who painted each one. The people who had studied the mixed set were better at it. Yet when asked afterwards, most of them said the blocked order had helped them more.

A meta-analysis

A 2019 meta-analysis by Marion Brunmair and Tobias Richter pooled the studies to date. Overall, interleaving helped. But how much depended on the material: the benefit was largest for things that look alike and are easy to confuse, such as paintings by different artists, smaller for maths, and absent or even negative for some materials, such as lists of words.

Why mixing helps

Blocked practice tells you which method to use before you start. If every problem on the page is about cones, you never have to ask whether this one is a cone problem. An exam doesn't tell you. Interleaving makes you practise that choice, over and over, and makes you notice the features that set one type apart from another.

The same goes for categories. Studying beta-blockers, then ACE inhibitors, then calcium channel blockers, each in its own block, never asks you to tell them apart. Mixing them does, and telling them apart is what the exam question will need.

Robert Bjork calls effort like this a desirable difficulty: it makes practice harder and slower, and it makes learning last. It's also why interleaving is so rarely used. It feels like it isn't working, exactly as the painting study showed.

When it doesn't help

• When you haven't learned the basics yet. Learn a new method in a short block first, then mix it in once you can do one example on your own.
• When the topics have nothing in common. Mixing anatomy with French verbs doesn't teach you to tell anything apart; it only switches context.
• When there's no choice to make. If every problem in a course uses the same method, mixing adds little.
• When you're learning simple word lists. The meta-analysis found little benefit, or a cost, for words. Spaced flashcards suit vocabulary better.

How to interleave your revision

1. Learn each new method or topic in a short block first, until you can do one example without help.
2. Then mix: build practice sets from the last few chapters, shuffled, so you don't know which topic comes next.
3. Before solving, name the type: “This is a related-rates problem”, “This is a calcium channel blocker, not a beta-blocker”. That decision is the skill you're training.
4. Put the things you confuse side by side on purpose: similar drugs, similar diagnoses, similar legal tests, similar tenses.
5. Use past exam papers. They're already interleaved, and they show how topics are mixed in your course.
6. Review your flashcards across topics in one session, instead of one deck at a time.

Examples by subject

Interleaving with flashcards

A spaced repetition scheduler interleaves for you. The cards due today come from different lectures and weeks, so reviewing all of a course's due cards together, rather than one lecture at a time, gives you mixing for free.

Cards that compare two things are interleaving inside a single card: “How does a beta-blocker's effect on heart rate differ from a dihydropyridine's?” FlashCardz writes cards like these when you miss one. The next night the idea comes back from a new angle, and one of those angles sets it against what it's easily confused with. See how the AI flashcard generator works.