Interleaving

What is interleaving?

Interleaving is practising several related skills or categories in a mixed order, so that one item after another calls for a different answer or method. The usual alternative, blocked practice, finishes all the items of one type before moving on to the next.

A typical maths textbook shows the difference. A chapter on the volume of a cone ends with a dozen cone problems, so you know which formula to use before reading each one. An interleaved assignment mixes cone problems with problems on spheres, cylinders and earlier topics, so you first have to work out which formula each problem needs.

Does interleaving work better than blocked practice?

Often yes: in maths, category learning and motor skills, interleaved practice has usually led to better scores on delayed tests than blocked practice, even though it went worse during practice.

• Maths. College students who practised four kinds of volume problems in a mixed order did much better on a test a week later than those who practised them in blocks.^[1] In a randomized trial in 54 seventh-grade classes, students who received interleaved assignments over four months outscored those with blocked assignments on a surprise test a month later, 61% to 38%.^[2]
• Categories. People who studied paintings by 12 artists in a mixed order were better at naming the artist of paintings they had never seen than when each artist's paintings were shown together.^[3]
• Motor skills. People learning three arm movements in a random order performed worse during practice than those practising them in blocks, but better on later retention and transfer tests.^[4] Sport scientists call this the contextual interference effect.

A 2019 meta-analysis, which pooled 59 studies, found a moderate benefit overall: largest for paintings and other visual material, smaller for maths.^[5]

Why does interleaving work?

The main explanation is that mixing makes you notice what sets one type apart from another and pick the right method, a step blocked practice lets you skip. In a study where children practised four kinds of maths problems with the same amount of spacing in both conditions, interleaving doubled their scores on a test the next day, and their errors suggested they had become better at matching each problem with its procedure.^[6]

With paintings, showing several artists' works side by side helped as much as interleaving them, while adding time between paintings by the same artist did not help.^[7] Researchers still debate the details. One account holds that interleaving draws attention to differences between categories, and blocking to what members of the same category share, which would explain why blocking sometimes wins.^[8] Spacing can add a benefit of its own, as long as it does not separate the items being compared.^[9]

Is interleaving just the spacing effect?

No, but the two usually come together: when you interleave, the items of each type are spread out, so interleaving also spaces practice.^[6] That makes many studies hard to read, because the benefit could come from the spacing effect alone.

Studies that separated the two found that the mixing itself matters. Interleaving still helped when spacing was held equal,^[6] and spacing out paintings by the same artist did not help on its own.^[7] For a learner, the distinction matters less: a schedule that mixes types and comes back to each one over several days gets both benefits.

When does interleaving help, and when does it not?

Interleaving helps most when the things you mix are similar enough to be confused, and little or not at all when they are clearly different.

• Similar categories: helps. The 2019 meta-analysis found larger benefits when the categories were similar to each other and when examples within a category varied.^[5] A 2021 systematic review also found interleaving most useful when the differences between items are subtle.^[10]
• Very different categories: can backfire. When the categories were clearly different from each other and their members had little in common, blocked study led to better results.^[8]
• Words: blocking did better. In studies of word learning, blocked practice beat interleaving on average.^[5]
• Unrelated subjects: no clear gain. Mixing flashcards on anatomy and Indonesian vocabulary made no reliable difference compared with studying one subject and then the other.^[11]
• Complete beginners: unclear. In the maths studies, students first learned each method in a tutorial, and the mixing applied to the practice afterwards.^[1] Most studies tested university students in the lab, so the evidence for school settings is thinner.^[10]

Why does blocked practice feel more effective?

Blocked practice feels more effective because you get more answers right while doing it: the method is fresh and you repeat it, while interleaving produces more errors.^[1] Learners take that ease as a sign of learning.

The feeling misleads. In the paintings study, 78% of participants learned better with interleaving, yet 78% said blocking had worked as well or better.^[3] Interleaving is one of the desirable difficulties: conditions that slow practice down but improve what you keep afterwards.

How do you use interleaving when you study?

Mix problem types or categories you have already learned, so that each item makes you decide what it is before you answer.

• Learn first, then mix. Work through a new method on its own, then mix it with earlier ones in later practice.
• Mix what you confuse. Similar formulas, drug classes, verb tenses, bird species or painters' styles benefit most.
• Shuffle your problem sets. Past exam papers already mix topics. You can also take a few problems from each of several chapters and do them in random order.
• Name the type first. Before solving, say which kind of problem it is and which method it needs. That is the skill interleaving trains.
• With flashcards. A spaced repetition review session usually mixes the cards that are due, which combines interleaving with active recall. In Anki, the default review order is by due date, then random, and studying a parent deck draws cards from its subdecks. The Anki manual warns that showing material in the same order every time makes answers easier to guess from context.^[12] Keep cards you tend to confuse in the same deck. Mixing unrelated decks is unlikely to add much.^[11]