Teacher of psychology, author, researcher.

Memory & Education Blog

A blog about education, psychology, and the links between the two.

Interleaving - using it in the classroom

Interleaving images of animals can be a more effective way to learn what a species looks like than ‘blocking’ examples. Image via Pixabay.com

Interleaving images of animals can be a more effective way to learn what a species looks like than ‘blocking’ examples. Image via Pixabay.com

There have been a lot of discussions about interleaving and spacing on Twitter this week, which is great. It’s always good to see teachers engaging with research, asking questions, and trying to make their practice evidence informed.

Some key principles to be aware of

As I have just been working on a systematic review of research into interleaving, I though I’d share a couple of things that I think are easily overlooked when people apply interleaving to their classroom practice (including by me, when I first came across the idea!).

- Interleaving seems to be beneficial because it allows learners to contrast different problems or examples, and/or because they pay more attention (they mindwander less) compared to seeing repetitive types of problems or examples. Let’s say you wanted to learn the difference between hawks and falcons. It would be preferable to see interleaved examples of hawks and falcons (HFHFHFHF) rather than several examples of hawks (HHHH) followed by several examples of falcons (FFFF; Eglinton & Kang, 2017). The latter would be called a ‘blocked sequence’.

- Interleaving, at least as it is defined in the research literature, relates to short items — specific questions, one-sentence examples, images. At most, a couple of paragraphs of text (a few studies have interleaved case studies of types of psychological disorders). It doesn’t mean interleaving entire lessons or topics (though that is, of course, also possible, and in the case of lessons it already happens in most school timetables).

- Similarly, the research evidence as a whole does not tend to study blocks that occur in different study sessions (e.g. hawks on Monday, falcons on Tuesday), as might happen in schools. It seems likely to me that this would increase the advantage of interleaving, because it would make it even more difficult to contrast the different types of item in the blocks. However, the literature as a whole is a bit short of studies of interleaving in real school contexts, so we await further evidence on this!

- Spacing, on the other hand, is often studied over long timescales, such as from one lesson to the next. The spacing effect means that people learn better if there is a longer delay between one study session and the next. Spacing could mean seconds, minutes, weeks, or even years (the benefit of longer delays is sometimes referred to as ‘the lag effect’).

- If you interleave some examples, there is inevitably a slight delay i.e. spacing between one example and the next one of the same type. Initially, that is why researchers thought that interleaving was helpful (e.g. Kornell & Bjork, 2008)*, but this turned out not to be the case; if you space out a blocked schedule by adding filler items, the interleaved examples still lead to better learning.

- Although spacing and interleaving are both desirable difficulties, this doesn’t mean that it it’s best to do both at once. In fact, spacing out interleaved examples can make things worse - it makes that contrast between one item and the next more difficult, meaning that the learning is no better than blocking (Birnbaum et al, 2013; Kang & Pashler, 2012). As Monica Birnbaum and colleagues put it, “two desirable difficulties are not always more desirable than one“ (p. 401).

- You can interleave either new learning, or practice tasks. I talk about that issue more here. In both cases, discrimination between different types of example is important.

Some classroom examples

Since interleaving is all about contrasting examples, let’s do a bit of meta-interleaving (I might have just invented that term!) with examples that feature interleaving, retrieval practice, and spacing:

1. Teacher A shows a class examples of three types of rock (igneous, metamorphic, and sedimentary)**. They cover igneous at the start of the class for a few minutes, then look at examples of metamorphic rock, and then examples of sedimentary.

Comment: this is a blocked presentation. There is no spacing and no retrieval. It would be preferable to interleave the different examples.

2. Now Teacher A shows up a mixture of different types of rock on a Powerpoint, and asks the class what type they are.

Comment: this is an interleaved practice task. It also involves retrieval of what has previously been learned. It’s not spaced to any meaningful extent.

3. Teacher B does the same presentation, but delays the practice task by half an hour, during which time the class writes a short essay.

Comment: the presentation is still blocked, but the practice task can now benefit from spacing as well as interleaving and retrieval.

4. Teacher C covers igneous rocks, then does half of the essay task, then covers metamorphic rocks, then does the other half of the essay task, then covers sedimentary rocks.

Comment: this is an unhelpful use of interleaving and spacing, because the delays are making it harder for learners to contrast the three types of rock.

5. The next day, Teacher C shows the mixed practice task, and calls out the types of rock rather than asking the class to identify them.

Comment: here, there is some spacing (over a day) but no retrieval practice, as the information is given to the pupils again (rather like spaced re-reading) rather than asking them to remember it. There is still interleaving, which could be helpful.

Some simple recommendations

Hopefully these examples show that we can have various combinations of retrieval, spacing, and interleaving. All three can be helpful if used appropriately. In general, interleaving means promoting contrast. The key rule of thumb, for either interleaved learning or practice, is:

- Mix up different questions/examples, but do so within the same study session.

Spacing and retrieval can both be usefully combined with interleaving (as seen in example 3, above) or used separately. It’s useful to remember that spacing out practice, for all that it helps tackle forgetting, can have a detrimental effect on learners’ ability to contrast examples.

One final point - interleaving is only helpful if the content (the different examples/items) is related in some way. The more similar they are (like the different types of rocks and birds) the more it is likely to help learners make these contrasts. An example in the domain of English or languages would be to contrast easily-confused grammatical structures, genres or author styles. It’s unlikely to help if you interleave English with Biology.

Any questions, feel free to comment here or ask me on Twitter, @JW_Firth. For more about how we can apply memory research to education, check out my co-authored book “Psychology in the classroom: A teacher’s guide to what works”, and my book for pupils/students, “How to learn”.


Birnbaum, M. S., Kornell, N., Bjork, E. L., & Bjork, R. A. (2013). Why interleaving enhances inductive learning: The roles of discrimination and retrieval. Memory & Cognition, 41(3), 392-402.

Eglington, L. G., & Kang, S. H. (2017). Interleaved presentation benefits science category learning. Journal of Applied Research in Memory and Cognition, 6(4), 475-485.

Kang, S. H., & Pashler, H. (2012). Learning painting styles: Spacing is advantageous when it promotes discriminative contrast. Applied Cognitive Psychology, 26(1), 97-103.

Kornell, N. & Bjork, R. A. (2008). Learning concepts and categories: Is spacing the “enemy of induction”? Psychological Science, 19, 585-592.

Yan, V. X., Soderstrom, N. C., Seneviratna, G. S., Bjork, E. L., & Bjork, R. A. (2017). How should exemplars be sequenced in inductive learning? Empirical evidence versus learners’ opinions. Journal of Experimental Psychology: Applied, 23(4), 403-416.

* Confusingly, the early studies of interleaving often referred to it as ’spacing’ for this reason!

** The rocks example was suggested by Yan et al, 2017.

Jonathan Firth