Interleaving: Why Mixing Topics Beats Studying in Blocks

Learning science

Finishing one topic before you start the next feels tidy and efficient. A run of studies says that neat, blocked approach is quietly one of the weakest ways to practise — and that shuffling topics together, awkward as it feels, is one of the strongest.

Picture how most revision timetables are built. Monday is fractions. Tuesday is percentages. Wednesday is ratios. Each topic gets its own block, worked through to the end before the next one begins. It feels logical: master one thing, then move on. It also feels good, because by the fifth fractions problem in a row you are sailing through them.

That smoothness is the trap. When you drill the same kind of problem over and over, you stop solving each one from scratch — you just repeat the move you made a moment ago. The practice gets easier because you are doing less thinking, not more learning. Cognitive scientists have a name for the alternative that fixes this: interleaving, or mixing different problem types together in a single session.

The obscure-solids experiment

The clearest demonstration comes from Doug Rohrer and Kelli Taylor. College students learned to compute the volumes of four unusual solids. One group practised in blocks: all the problems for the first solid, then all for the second, and so on. The other group got the exact same problems interleaved — the four types shuffled together so no two consecutive problems were the same kind.

During the practice session, the blocked group looked far better. They answered more problems correctly and clearly felt more fluent. If you had walked in and watched, you would have bet on them. Then came a test a week later — and the result flipped completely. The interleaved group vastly outperformed the blockers, despite having looked worse while practising. Same problems, same time on task; only the order changed.

Blocked practice makes you better at the practice. Interleaved practice makes you better at the test.

Why mixing works

The benefit comes down to a skill that blocked practice never asks for: choosing the right approach. When every problem on the page is a fractions problem, you never have to decide that it is one — the heading already told you. In the real exam, and in real life, nothing tells you. The hardest part is often recognising which kind of problem you are looking at and which method it calls for.

Interleaving forces exactly that decision on every single problem. Because the next question could be anything, you have to stop, read it properly, and retrieve the right strategy before you can even begin. Rohrer calls this learning to “discriminate” between problem types — and it is precisely the skill that transfers to a test, where the questions arrive in no particular order.

It also quietly builds in another proven advantage. Mixing topics means you naturally return to each one after a gap rather than in one solid run, so interleaving smuggles in a dose of spaced practice — another of the most reliable findings in the whole of learning research.

It holds up in a real classroom

A fair worry is that this only works for tidy lab tasks with made-up solids. So Rohrer and colleagues ran it where it counts: an actual school. Over a term, students worked through maths practice that was either blocked or interleaved, using their ordinary curriculum. At the end they sat a surprise test.

The interleaved students came out well ahead — a large, practically meaningful gap, not a statistical footnote. Reviews of the wider literature reach the same broad verdict: interleaving reliably helps, especially for the kinds of problems students most often confuse with one another. The effect is not magic and not universal, but the direction is consistent and the mechanism is well understood.

The catch you have to accept

Interleaving comes with a genuine cost, and pretending otherwise would be dishonest: it feels worse. You make more mistakes during practice, you move more slowly, and you finish a session less sure that you have “got it”. Robert Bjork groups this with his desirable difficulties — conditions that hurt performance now but help learning later. The discomfort is not a sign it is going wrong. It is the sign it is working.

This is also why so few people do it. Blocked practice offers a steady drip of small wins; interleaving offers struggle now and a payoff you cannot see until the test. Left to our instincts, almost all of us pick the version that feels better and learns worse.

How to interleave, in practice

Mix problem types within a session. Instead of twenty fractions questions, do a set that jumps between fractions, percentages and ratios. The jumping is the point — it forces you to identify each problem before solving it.

Shuffle, do not sort. When you build a revision set or a deck, resist the urge to group everything by topic. A deliberately jumbled order does more work than a neat one.

Interleave things that are easy to confuse. The benefit is largest for concepts that look similar and get mixed up — different grammar tenses, similar-sounding vocabulary, related formulas, artists from the same movement. Practising them side by side teaches the differences.

Expect it to feel harder — and trust the test. Judge the method by what you can recall days later, not by how smooth the session felt. Smoothness is the feeling you are trying to give up.

Where a quiz fits

A quiz is interleaving with the awkwardness built in for free. A good round hops from topic to topic by design: one question on this, the next on something else entirely, nothing grouped, nothing signposted. Every question makes you work out what it is actually asking before you can answer — which is exactly the discrimination skill the research says matters.

That is part of why Yippito lets you turn a whole topic — or several — into one mixed quiz in seconds. Instead of drilling one idea into the ground, a class or a group of friends gets a shuffled set that keeps everyone genuinely thinking on every question. It feels like a game. It works like interleaved practice.

Sources

• Taylor, K., & Rohrer, D. (2010). The Effects of Interleaved Practice. Applied Cognitive Psychology, 24(6), 837–848.
• Rohrer, D., Dedrick, R. F., & Stershic, S. (2015). Interleaved Practice Improves Mathematics Learning. Journal of Educational Psychology, 107(3), 900–908.
• Rohrer, D., Dedrick, R. F., & Hartwig, M. K. A Guide to Interleaved Mathematics Practice (University of South Florida).