Interleaving: why mixing topics helps you study

What is interleaving?

Interleaving means mixing different topics or problem types within one study session, instead of finishing one before moving to the next. The usual approach, called blocked practice, looks like AAA BBB CCC: ten problems on topic A, then ten on B, then ten on C. Interleaved practice looks like ABC BCA CAB: the same problems, shuffled.

The difference matters because in an exam, the questions don't come labelled with the chapter they belong to. Blocked practice tells you which method to use before you've read the problem. Interleaved practice makes you work that out each time, which is the skill the exam actually tests.

What the research found

In one well-known experiment, 18 undergraduates learned to calculate the volume of four unfamiliar geometric solids (Rohrer & Taylor, 2007). One group practised in blocks, one solid at a time. The other did the same problems mixed together. During practice, the blocked group scored higher: 89% against 60%. On a test one week later, the result flipped. The mixed group scored 63% and the blocked group 20%.

The effect also holds in real classrooms. In a study of 126 seventh-grade students over three months, interleaved maths practice produced higher scores than blocked practice on a test 1 day later and on a test 30 days later, with effect sizes (Cohen's d) of 0.42 and 0.79 (Rohrer, Dedrick & Stershic, 2015). The gap grew with time.

It isn't only maths. When people studied paintings by several artists, seeing each artist's work interleaved with others helped them later recognise the artist of new paintings they hadn't seen (Kornell & Bjork, 2008). A meta-analysis pooling 59 studies found a moderate overall interleaving effect, Hedges' g = 0.42 (Brunmair & Richter, 2019).

Why mixing topics works

With blocked practice, you know which method to use before you start: every problem in the set is about the same thing. Interleaved practice requires you to choose a strategy based on the problem itself, as you must in a final exam that covers the whole course, as Rohrer and colleagues point out. You practise recognising what kind of problem you're facing, not just carrying out the steps.

Mixing also forces comparison. Seeing a cardiac muscle slide right after a skeletal muscle slide makes you notice what tells them apart. Ten cardiac slides in a row never do.

Where interleaving helps most, and where it doesn't

The 2019 meta-analysis found that interleaving effects were stronger when categories were similar to each other and hard to tell apart, and for more complex material. It also found an advantage for blocking in studies using words, and unclear results for expository texts, the kind of reading where you learn from prose (Brunmair & Richter, 2019). The authors conclude that interleaving should be used with caution in those conditions.

• Good fit: problem types that look alike but need different methods (types of statistical test, kinetics problems, genetics crosses).
• Good fit: visual categories you must tell apart (tissue types in histology, ECG rhythms, bird species).
• Good fit: diagnoses or conditions with overlapping signs.
• Weaker fit: memorizing vocabulary or isolated facts.
• Weaker fit: a first pass through a new topic you don't understand yet.

That last point matters. Learn each new method on its own first, so you know how it works. Then mix.

Interleaving and spacing: not the same thing

Interleaving and spacing often get confused, partly because they go together. Spacing means returning to a topic after a gap. Interleaving means mixing topics within a session. When you interleave, each topic naturally comes back after a gap, so you get some spacing along the way. But the benefit of interleaving comes from comparing and choosing between topics, which spacing alone doesn't give you. A 2013 review of study techniques rated distributed practice as high utility and interleaved practice as moderate utility, noting that research on interleaving had only begun to be systematically explored (Dunlosky et al., 2013).

A revision plan that mixes topics

Here's an example for a physiology module with four topics: the heart, blood vessels, the kidney and the lungs.

1. 1Week 1: one session per topic, blocked. Make sure you understand each one and write practice questions in each lecture note.
2. 2Week 2: each session takes questions from two topics, alternating: heart and kidney, then vessels and lungs.
3. 3Week 3: every session draws from all four topics in shuffled order.
4. 4Final week: past papers or mixed question sets only, timed, in exam conditions.

How to interleave from your own notes

• Collect the questions from several lecture notes into one list, then shuffle it. A deck of index cards works; so does a numbered list and a random number.
• Hide which lecture each question came from. Write the source on the back, or at the bottom of the list.
• Before answering, say which topic or method the question needs. Getting that right is half the skill.
• Check each answer against the original note, and mark the ones you got wrong for the next mixed session.

Mixing questions with an AI notes app

Building mixed question sets by hand takes time. With Lumyo, you can choose which notes a chat reads, for example four lectures from different weeks, and ask it to quiz you with questions that mix them. Answer first, then check: each answer cites the notes it came from, and tapping a source opens the exact line. For more on self-testing, see how to study from your own notes before an exam.