Ace
Short answer: interleaving means mixing different types of problems or topics in one practice session instead of doing them in blocks. It feels slower and you make more mistakes while practising, but on a later test students who interleave tend to do better, because they learn to recognise which method each problem needs. Use it after you've learned each topic once, with related topics you could confuse.
What is interleaving?
Most textbooks and problem sheets are blocked: ten questions on one method, then ten on the next. Interleaving shuffles them: a question on topic A, then C, then B, then A again. The topics are the same; only the order changes.
The point isn't variety for its own sake. In an exam, questions don't arrive labelled with the method you need. Blocked practice lets you skip the step of working out which method applies, because you already know. Interleaved practice makes you do that step every time, which is exactly what the exam asks.
What does the research show about interleaving?
A classic study by Rohrer and Taylor (2007) had college students practise calculating the volumes of different shapes. Some practised in blocks, others with the problem types mixed. On a test a week later, the students who had practised the mixed version did far better.
The large review of study techniques by Dunlosky and colleagues (2013) rated interleaved practice as moderately useful: promising, with strong results in maths and some other areas, but less evidence across subjects than for its two top-rated techniques, practice testing and spacing. Interleaving works best combined with them, not instead of them.
Why does interleaving feel worse if it works better?
Because performance during practice and learning aren't the same thing. In Rohrer and Taylor's study, the blocked group did better during the practice sessions themselves, then worse on the later test. Blocked practice gives you a run of right answers because you're repeating one method. That feels like mastery. Mixed practice gives you more wrong answers, because each question first asks "what kind of problem is this?"
The same trap appears with spacing: in Kornell's (2009) flashcard experiments most students believed cramming had worked better, even though spacing beat it for most of them. If a session feels effortful and slightly messy, that's often a sign it's working. See active recall vs rereading for the same idea in another form.
How do you interleave in different subjects?
- Maths and physics: once you've learned three or four methods, do mixed problem sets. Cover the chapter headings on a past paper and pull questions from across it. Our guide to how to study maths has more on this.
- Sciences: mix questions from related topics students confuse: mitosis and meiosis, aerobic and anaerobic respiration, ionic and covalent bonding.
- Languages: mix tenses or cases in one session rather than drilling one at a time.
- Essay subjects: practise planning answers to questions from different parts of the course in one sitting, so you learn to choose which material each question needs.
- Your timetable: two or three subjects a day instead of one. See how to make a revision timetable.
Practice questions make interleaving easy, because you can shuffle them. Ace turns your own lecture slides, PDFs or YouTube lectures into practice questions and lets you pick which chapters go into a set, so you can build a mixed set across topics, with the line from your notes shown behind every answer.
When shouldn't you interleave?
- When something is brand new. Learn a method on its own first. Interleaving helps you choose between methods you already know.
- With unrelated topics. Mixing organic chemistry with medieval history adds switching cost without the "which method?" benefit. Mix things you could confuse.
- The night before. Last-minute review is for your weakest topics; interleaving pays off over weeks.
Frequently asked questions
What is an example of interleaving?
Instead of ten area problems then ten volume problems, you do them mixed: area, volume, volume, area, surface area, and so on. Each question first makes you decide which method it needs.
Is interleaving better than spaced repetition?
They do different jobs and work best together. Spacing decides when you come back to a topic; interleaving decides what you mix with it in a session.
Does interleaving work for every subject?
The strongest evidence is in maths and other problem-solving subjects. It's most useful wherever you need to tell similar things apart.
Sources
- Rohrer, D. & Taylor, K. (2007). The shuffling of mathematics problems improves learning. Instructional Science, 35(6), 481–498. Record
- Dunlosky, J., Rawson, K. A., Marsh, E. J., Nathan, M. J. & Willingham, D. T. (2013). Improving students' learning with effective learning techniques. Psychological Science in the Public Interest, 14(1), 4–58. APS summary
- Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297–1317. Record
