What to interleave
What I used to think: You should interleave different subjects.
What I now think: You should interleave concepts within a topic, especially ones that students tend to get confused by.
As I had previously thought that Interleaving was similar to Spacing, it would make sense to interleave subjects. However, if the benefit is primarily on helping students compare and contrast “similar but different” things, then it would make more sense to focus on ideas, concepts and events that have a similar classification.
For example, instead of trying to interleave Maths, French and Geography, it would make sense instead to interleave specific examples from within the same subject. Within Politics, this could mean interleaving the end of Margaret Thatcher’s and Boris Johnson’s respective stewardship of Prime Minister. In doing so, students would be able to identify what these two events had in common (i.e. they did not leave office due to losing a general election) and what the key differences between them were (i.e. the roles their respective Chancellors of the Exchequer played in their final weeks).
When to interleave
What I used to think: You can interleave your curriculum.
What I now think: It’s better to interleave within an individual lesson.
This is because proximity of content is key. The closer in time and space the things that you are interleaving are, the easier it is to remember the similarities and differences between them. If the gap is too long, misremembering is more likely to occur.
That being said, I think there is a lot to be said for factoring in Spacing when it comes to the curriculum. Revisiting material regularly, especially when students have to remember large amounts of information for a long period of time, is essential. But for specific interleaving, within lesson is probably more appropriate.
Interleaving is only relevant for some subjects
What I used to think: Interleaving is only for some specific subjects (i.e., Maths).
What I now think: Although some subjects may be more naturally suited to Interleaving (as well as being more impactful), I believe you can do Interleaving within any subject.
When running CPD for teachers on the science of learning, a common question I get on Interleaving is that it feels more intuitive for some subjects than others. Typically, subjects like Maths are in the former category, and subjects like History (which is typically more chronologically based) or Art (which is more skills based) in the latter.
I used to go along and agree with this, as I didn’t want to rock the boat too much. However, I think this belief was based on me having offered a poor explanation as to what Interleaving is. If you view it as similar to spacing, that would make sense.
However, I now believe every subject can apply Interleaving. Every subject contains areas that can be compared and contrasted with one another. There are always times when you would choose to do X instead of Y. Interleaving can help students to become aware of the thought processes as to when those situations may be.
Who is in charge of the Interleaving
What I used to think: Students should interleave their revision.
What I now think: Teachers are better placed to do Interleaving within their lesson. An easier and bigger win for students when studying independently would be to focus on Spacing and Retrieval Practice.
Because Interleaving involves uncovering misconceptions as well as identifying similarities and differences, it often needs some guidance. Self-selecting what to interleave is inherently very difficult. Having an expert (i.e., the teacher) on hand to tease out the contrasts really helps. This means that I think it is an area of cognitive science that can be helpful in the classroom, but less so during independent study. When it comes to revision, I think students are on much safer ground prioritising Retrieval Practice and Spacing.
bonus – The optimal number of things to interleave
What I used to think: There is no optimal number of items to interleave, but if there was, it would probably be 3-4, as this had some sort of link to working memory capacity
What I kind of now think: I still don’t know if there is an optimal number, but if in doubt I’d say focus on just two items (especially for novice learners).
This is because working memory is way more fragile than I had previously appreciated and arguably makes discrimination learning easier and more apparent. That being said, I am still open to changing my mind on this one as more research emerges.
When it comes to applying cognitive science to education, I fully believe that no-one knows anything 100% for sure. We are all just guessing. Some guesses are better and more informed than others. We can be more confident about general principles, as opposed to specific strategies. This means that mistakes will be made along the way, but we should embrace that and accept that changes in what we think are inevitable.
I have no doubt that I will look back in a few years and reflect that what I currently think has changed. As Muhammad Ali once said: “