Interleaving and variation

In my last post I discussed the benefits of spacing out study, however another potent alternative to massed practice is interleaving, which involves mixing up the order of materials across different topics. This will impede performance during initial learning but boost final test performance. Compared to massed practice, a significant advantage of interleaving is that it helps us learn better how to assess context and discriminate between problems, selecting and applying the correct solution from a range of possibilities. The theory is that interleaving requires learners to constantly “reload” motor programs (in the case of motor skills) or retrieve strategies/information (in the case of cognitive skills) and allows learners to extract more general rules that aid transfer. In ‘Make it Stick: the science of successful learning’, Roediger, McDaniel & Brown state:

‘For our learning to have practical value, we must be adept at discerning ‘what kind of problem is this?’ so we can select and apply an appropriate solution. Strategies of learning that help students identify and discern complex prototypes can help them grasp the kinds of conceptual and functional differences that go beyond the acquisition of knowledge and reach into the higher sphere of comprehension.’

In this clip Robert Bjork, distinguished Professor of Psychology at the University of California, discusses the benefits of interleaving:

It is also recommended that practice is varied. For example, tossing bean bags into baskets at mixed distances, improves your ability to transfer learning from one situation and apply it successfully to another. You develop a broader understanding of the relationships between different conditions and the movements required to succeed in them. Additionally, context is discerned better and a more flexible ‘movement vocabulary’ is developed – different movements for different situations. Varied and more challenging practice demands more brain power and encodes the learning in a more flexible representation that can be applied more broadly

How do we know interleaving is effective?

Although Dunlowsky et al, in a study of effective study strategies by  in 2013, said they would like to see additional research into interleaving,  they did consider it promising in improving student learning. The evidence base for the effectiveness of interleaving is certainly building, for example, benefits have been found for word pairs (Battig, 1979), motor movements (Shea & Morgan, 1979) and word translations (Richland, R. A. Bjork, & Finley, 2004). Carson & Wiegand (1979) also found that interleaving benefits not only memory for what is studied, but also leads to benefits in the transfer of learned skills. In 2007 Rohrer & Taylor led a study in which students took part in two practice sessions (separated by a week), utilizing either had massed practice or interleaved practice to learn to compute the volume of four different geometric solids. For massed practice, students practiced each problem one at a time before moving onto the next.  For interleaved practice, students never practiced the same kind of solid consecutively; they practiced solving for the volume of a wedge, followed by a spherical cone, followed by a spheroid, and so forth, until they had practiced four problems of each type. Dunlowsky, in his article entitled ‘Strengthening the Student Toolbox’, describes the results.

The results presented in Figure 1 (on the right) show thinterleaving studyat during the practice sessions,
performance finding the correct volumes was considerably higher for massed practice than for interleaved practice, which is why some students (and teachers) may prefer massed practice. The reason not to stick with massed practice is revealed when we examine performance on the exam, which occurred one week after the final practice session. As shown in the bars on the far right of Figure 1, students who massed practice performed horribly. By contrast, those who interleaved did three times better on the exam, and their performance did not decline compared with the original practice session! If students who interleaved had practiced just a couple more times, no doubt they would have performed even better, but the message is clear: massed practice leads to quick learning and quick forgetting, whereas interleaved practice slows learning but leads to much greater retention.

How can teachers use interleaving in their classrooms?

  • When creating practice tests for students (whether to be completed in class or at home), it is best to mix up problems of different kinds. even though students initially may struggle a bit more, they will benefit in the long run.
  • Rather than just practicing math rules say for Algebra from the previous session, practiced the rule from the previous session intermixed with the practice of rules from even earlier sessions
  • Many traditional math textbooks encourage mass practice because each chapter deals with similar problems. However, interleaving would require the solving problems from different types, such as adding, subtracting, multiplying, and dividing consecutively.
  • In History, rather than spending six weeks studying one topic, such as the unification & consolidation of Germany, mix it with another. This might be a study of Russia around the same period, which would allow for consideration of the underlying factors that drove change at this time.
  • Make sure the mini skills you interleave are related in some higher-order way. If you’re trying to learn tennis, you’d want to interleave serves, backhands, volleys, smashes, and footwork—not serves, synchronized swimming, European capitals, and programming in Java.
  • In World Languages base units on a context rather than a topic. For example, units about history or geography or even art (‘Dans la jungle’ and ‘L’art’). One MFL department has a unit based on the documentary ‘Maradona by Kusturica’ which involves different topics such as health, personal relationships, home environment and social issues. These units are interleaved, covering various topics within each of them.


Practice, practice, practice brings momentary strength, but not ‘underlying habit strength’. The very techniques that build habit strength, like spacing, interleaving, and variation, however, slow improvement. This presents teachers with an problem: If the benefits of this strategy are not made clear it might lead to demotivation and a decline in efforts, as the progress is imperceptible. But, the evidence is clear. The simple act of spacing out study and practice in installments, allowing time to elapse between them, makes both the learning and the memory stronger, in effect building habit strength. Furthermore, interleaving helps you develop your ability to discriminate later between different kinds of problems and select the right tool. Practice that’s spaced out, interleaved with other learning, and varied produces better mastery, longer retention, and more versatility.

Readings and websites

Strengthening the student toolkit by John Dunlowsky.

Make it Stick by Roediger, McDaniel & Brow

Bjork Learning and Forgetting Lab. Robert Bjork.

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