A 2010 study by Kelli Taylor and Doug Rohrer found that students who practised four types of maths problems mixed together scored roughly double, on a test the next day, compared to students who practised the same problems blocked by type. During the practice session itself, the mixed group actually performed worse โ€” more errors, slower, more visibly effortful. The paradox: the practice that felt more productive in the moment produced the worse outcome, and the practice that felt harder produced the better one.

The study, and why the result is counterintuitive

Taylor and Rohrer had children practise four kinds of geometry problems (each requiring a different formula). One group worked through them blocked โ€” all of problem type A, then all of type B, and so on. The other group worked through them interleaved โ€” the four types mixed in random order throughout the session.

During practice, the blocked group looked like they were doing well: fewer errors, faster completion, that satisfying feeling of "I've got this." The interleaved group looked worse in real time: more mistakes, more hesitation, more visible struggle. Then, on a test given the next day, the result flipped hard โ€” the interleaved group scored roughly double the blocked group.

Why mixing the order makes such a difference

Blocked practice lets a student solve every problem in a block the same way, because they already know which method applies โ€” it's the one they just used nineteen times. This trains execution (can you carry out procedure X) but not selection (can you recognise that procedure X is the one this problem needs). A real test โ€” or a real KS3 exam paper โ€” rarely tells a student which method to use. It requires them to work that out first.

Interleaved practice forces exactly that selection skill, every single question, because the method that worked last time might not work this time. That's genuinely harder in the moment โ€” hence the worse practice-session performance โ€” but it's also the actual skill being tested later. This is a specific case of what psychologist Robert Bjork calls a desirable difficulty: a difficulty that slows performance during learning but improves it afterward, because it forces more effortful, generative processing rather than passive repetition.

Why this matters more at KS3 than it looks like it should

A lot of KS3 homework and revision material is structured as blocks by design โ€” a worksheet titled "Solving Quadratic Equations," with twenty near-identical questions. That's not wrong as a first step when a technique is brand new. The risk is stopping there. If a student never practises quadratics mixed in with the other techniques they'll need to distinguish it from on exam day, the worksheet has trained execution without selection โ€” and the exam largely tests selection.

This is also the mechanism behind a specific, common parent complaint: "they seemed to really get it in the lesson." Lessons are often blocked by nature โ€” today's lesson is about one technique. The gap between lesson performance and exam performance frequently isn't a knowledge gap; it's a selection-skill gap that blocked practice never trained.

What "it felt easy" should tell you

Because interleaved practice produces more visible struggle in the moment, a revision session that feels smooth and easy is not necessarily a good sign โ€” it may mean the practice is blocked, testing execution the student has already over-learned rather than the selection skill they actually need. Conversely, a session with more mistakes on mixed material may be doing more real work than the score in that moment suggests.

What good interleaved practice looks like

Not chaos โ€” deliberate mixing. A well-structured revision session on, say, algebra might rotate between expanding brackets, solving equations, and simplifying expressions within the same session, rather than exhausting one topic before moving to the next. The student has to identify the type of problem before solving it, every time โ€” which is closer to what an actual exam paper demands.

FAQ

What is interleaving in learning?

Practising several types of problem mixed together rather than one type repeated in a block. It feels harder in the moment but produces better long-term retention because it trains recognising which method fits which problem.

Why did my child do well on a maths worksheet but badly on the actual test?

Worksheets are usually blocked by topic, so the student only needs to remember the last method used. A real test mixes topics, requiring the student to first identify which method applies โ€” a skill blocked practice doesn't train.

Is interleaved practice always better than blocked practice?

For durable, transferable learning tested later, yes, research consistently favours it. When a technique is brand new, some initial blocked repetition to establish the mechanics before mixing it in is still reasonable.

Duke Harewood ยท founder, aitutors.me ยท Updated 11 Jul 2026.