Interleaving is a study method where you mix different topics or problem types in one session instead of finishing one topic before moving to the next. Rather than doing 20 questions on topic A and then 20 on topic B, you alternate A, B, C, A, C, B. It feels harder and slower, but research on maths and other skills finds it improves performance on later tests.
Blocked versus interleaved practice
| Blocked | Interleaved | |
|---|---|---|
| Order | AAAA BBBB CCCC | ABCA CBAB CABC |
| During practice | Feels smooth, fast progress | Feels messy, more errors |
| On a later test | Often weaker | Often stronger |
| Best used | First meeting with a new idea | After you know the basics |
Why mixing helps
When all problems in a set use the same method, you can switch your brain off: you know the method before reading the question. In an exam, no one tells you which chapter a question comes from. Interleaving makes you do two things each time: identify what kind of problem it is, then apply the right method. That choice is a skill, and blocked practice never trains it.
It also spaces out each topic as a side effect. Returning to a topic after other work means you have to retrieve the method again rather than reuse what is still in your head.
What the research shows
In a well-known 2007 study, Doug Rohrer and Kelli Taylor had students learn to calculate the volume of solids. One group practised in blocks and the other mixed the problems. During practice the blocked group did better, but a week later the mixed group scored far higher on the test. The Dunlosky 2013 review rated interleaved practice as moderate utility, noting that most evidence comes from maths and motor skills and that more work is needed for other subjects.
How to interleave
- Learn each topic first. Use blocked practice to get the basic idea, because interleaving does not replace understanding.
- Collect problems from several topics. Use mixed past papers, textbook review sections or your own question list.
- Shuffle them. Cut up a list, number them and use a random order, or use the end-of-book mixed exercises.
- Name the type before you solve. Write down what kind of problem it is and why.
- Check answers straight away and note the confusion pairs: which two problem types do you keep mixing up?
- Repeat the mixed set a few days later with a different order.
Examples by subject
- Maths: mix quadratics, simultaneous equations and trigonometry questions in one set.
- Physics: alternate kinematics, energy and momentum problems.
- Languages: mix verb tenses in one exercise instead of one tense per page.
- Medicine: mix presentations of chest pain, abdominal pain and headache and decide the next step each time.
- Programming: write a loop, then a SQL query, then a recursive function, rather than one language feature for an hour.
- Essay subjects: alternate planning an answer on topic A, defining terms on B and recalling evidence for C.
A sample week
| Day | Session |
|---|---|
| Mon | Learn topic A (blocked), 5 practice questions |
| Tue | Learn topic B (blocked), 5 practice questions |
| Wed | Mixed set of A and B, 12 questions |
| Thu | Learn topic C (blocked) |
| Fri | Mixed set of A, B and C, 15 questions |
| Sat | Timed mixed past paper section |
Pitfalls
- Interleaving too early. If you cannot do any of the problem types yet, work through a few blocked examples first.
- Mixing unrelated subjects. The benefit shows most when topics look similar and are easy to confuse.
- Giving up because it feels hard. It is meant to feel harder. Judge it on a test a few days later, not on how smooth the session felt.
Questions about interleaving
Is interleaving the same as spacing? Not quite. Spacing is about the gaps between sessions on the same topic. Interleaving is about mixing different topics or problem types within a session. They work well together, and mixing topics tends to create spacing naturally.
Does it work for memorising facts?
The evidence is strongest for problem-solving skills and categories that are easy to confuse, such as painters' styles, bird species or maths problem types. For plain facts, such as a list of dates, spaced flashcards in a shuffled order do the same job.
What if my class teaches one topic at a time?
That is fine. Learn each topic when it comes, then build mixed sets from earlier weeks as you go. Revisiting old topics inside new homework is a simple way to keep them alive.
In Type to Study
Unit quizzes and the final exam in Type to Study draw on different units, and flashcards from several lessons come back in the same session, so your practice is mixed by default. If you study on paper, shuffling your question list does the same job.