Interleaving: why mixing topics beats studying one at a time

What is interleaving?

Interleaving is mixing different problem types, subjects or categories within one study session, as opposed to blocked practice where you finish all problems of one type before moving on. A blocked session looks like AAABBBCCC; an interleaved one looks like ABCBCACAB.

It is related to spaced repetition but is not the same thing. Spacing is about when you revise – distributing sessions over time. Interleaving is about what you do inside a session. They can be used separately, and they reinforce each other.

What does the research say?

Rohrer and Taylor (2007) is the standard reference. Students practised calculating the volume of four geometric solids, either blocked or mixed. During practice itself the blocked group did far better – 89 per cent correct against 60. One week later the ranking reversed: the interleaved group solved 63 per cent of the test problems, the blocked group 20.

Kornell and Bjork (2008) found the same pattern outside mathematics. Participants learned to recognise the styles of twelve painters, and those who saw the paintings interleaved classified new works better than those who saw one painter at a time. Strikingly, roughly eight in ten participants afterwards believed the blocked method had served them better – despite their own results saying otherwise.

In Dunlosky’s (2013) review of ten study techniques, interleaved practice is rated as of moderate utility, placing it above highlighting and rereading but below testing and distributed practice. That is an honest placing: the effect is robust within its domain but depends more on the material than the top two techniques do.

Why does it work?

Blocked practice lets you skip the hardest step. If you already know every problem on the page is the chain rule, you never have to work out that the chain rule applies – you just apply a formula you have just read. In the exam nobody tells you which rule applies, and that is where the training fails.

Interleaving forces two things. First, discrimination: you have to read the problem and identify its type. Second, retrieval: the method is not still sitting in working memory from the previous problem but has to be fetched afresh, which is the same mechanism that makes active recall effective.

Why does it feel worse when it is better?

Mixing creates what Robert Bjork calls desirable difficulties: obstacles that lower performance during practice but raise the learning measured afterwards. You make more mistakes, move more slowly and feel less confident – and the brain reads that feeling as "this is not working".

That is why the technique is rarely chosen voluntarily. We judge how well we are learning by how fluent it feels in the moment, which is a systematically unreliable gauge. A method that makes studying feel smooth – highlighting, rereading, blocked drilling – produces a strong sense of mastery and weak memory a week later.

Which subjects suit interleaving best?

The effect is largest when problem types resemble each other and are therefore easy to confuse. Then choosing the method is the genuinely hard part, and that is exactly what mixing trains.

So there is a limit. Learning two hundred vocabulary items with no risk of confusion gains little from mixing, and there distributed practice is the more important technique. How to set this up in technical modules is covered in our article on studying maths.

When should you not interleave?

Not when you meet a topic for the first time. You need to understand what the chain rule does before it is meaningful to practise recognising when it applies, and a mix of four half-understood methods produces only confusion.

The order that works is therefore: learn one method blocked until you can execute it, then start mixing as soon as the basics hold. Mixing should start earlier than most people think – usually after a handful of problems, not after a whole chapter.

How do you set up an interleaved session?

Take problems from three or four different sections and put them in random order rather than following the book’s chapter structure. Flipping pages is often enough: three problems from chapter 4, three from chapter 6, three from chapter 2, shuffled.

A concrete plan for an exam three weeks away: every session includes problems from everything covered so far, not just this week’s chapter. The share of older material should rise, not fall, as the exam approaches – the opposite of how most people organise their exam revision. Past papers, incidentally, are interleaved by nature, since their problems never arrive sorted by method.

How does interleaving fit with the rest of your study technique?

Interleaving is one of three techniques that measure well and hang together: you test yourself instead of rereading, distribute the sessions over time, and mix the content within each session. None of them costs more time than rereading – they only redistribute the time you already spend.

In Memmo, revision runs on FSRS, the same algorithm family Anki moved to, which means the interval between repetitions is computed per card from how you actually answered rather than following a fixed schedule. Mix cards from several modules or chapters into the same session and you get the interleaving for free. If your notes are digital you can turn them into flashcards and let sessions draw from several sections at once. We compare the approaches in our article on Anki, Quizlet and AI flashcards.