Bright Lines: How to Use Interleaving Effectively

The first in a series of posts that will look closely at how to apply the science of learning in a way that is clear, impactful and sustainable.

The Bright Lines Problem

I’ve been thinking a lot about the challenge of applying research to learning for a long time and it’s no secret that for me the best bets are from cognitive science, but I feel there is a danger now in people simply saying ‘the science of learning is great because science’ and applying it in a way that is at best having little impact on student achievement. We have much powerful knowledge now about how learning happens but the reality is we really don’t have bright lines yet on how to apply it in a way that is actually effective, scalable and most importantly, sustainable.

If we don’t take the rich evidence base we have and really consider and evaluate how we can use it in real learning environments, then the broader movement termed the ‘science of learning’ will be seen as a fad like so many other failed initiatives in education.

So this will be the first in a series of posts that will examine specific research findings through the lens of practical implementation, using real examples to illustrate both the promise and the pitfalls of evidence-based practice.

From Activities and Strategies to Principles That Guide Practice

I strongly believe now that we need to move from viewing the science of learning as a disconnected menu of strategies or activities to understanding it as a set of principles about how minds acquire, organise, and retrieve knowledge. Too often, evidence-based teaching is reduced to checklists: interleave, retrieve, space, elaborate etc. without considering how they interact and how they might determine long-term learning. Their effectiveness depends on the task, the content, and crucially, the learner’s prior knowledge. We don’t need more strategies, we need better explanations of when, why, and for whom they work.

Interleaving: What the Evidence Says

I’ve written a previous post here on the topic and why it’s really about boundary conditions but the essential idea is that interleaving works by forcing learners to actively discriminate between similar concepts, but only when they have the cognitive resources and prior knowledge to handle that discrimination

It’s one of the most robust findings in cognitive psychology and typically framed as a "desirable difficulty": harder in the short term, but better for long-term understanding, but recent research complicates that picture.

In a set of experiments, Jeri Little and colleagues show that the benefits of interleaving depend on what you want students to master and then the learner's strategy. Those who approach tasks by memorising examples perform better when materials are interleaved. But learners who try to abstract rules perform better when examples are blocked by category. In short, the optimal study sequence depends not just on the task, but on the to-be-learned material and as a result, how the student thinks.

Memorisation vs. Rule Abstraction

This has direct implications for education. Teachers should therefore consider not just what they teach, but how students are likely to learn. One-size-fits-all activities (even well-supported ones like interleaving) may miss the mark if they are not considered in the broader trajectory of student learning.

In the aforementioned study, participants learned to classify nonsense words into categories like "plant" or "animal." Some learners relied on memorising specific examples; others tried to extract an abstract rule (e.g. "words ending in -t are plants"). Crucially, the optimal study method changed depending on the strategy:

• Memorisers did better when examples were interleaved. They relied on surface similarity, and seeing examples from different categories helped them learn to discriminate.

• Rule-abstractors did better when examples were blocked. They benefited from seeing patterns within a category before comparing across categories.

What does this mean in real classrooms?

OK, so say you're trying to get students to learn how to distinguish between quadratic and linear equations. How you sequence problems can make a big difference. This is where the rubber meets the road: it's not just about whether to interleave, but when, why, and for whom.

Example 1. Quadratic vs. Linear Equations Blocked Practice

All problems are of one type before switching to the next:

1. Solve: x² - 5x + 6 = 0

2. Solve: x² + 3x - 4 = 0

3. Solve: x² - 2x = 0

4. Solve: 3x + 5 = 11

5. Solve: x - 7 = 2

Interleaved Practice: Mixing problem types:

1. Solve: x² - 2x = 0

2. Solve: 3x + 5 = 11

3. Solve: x² + 3x - 4 = 0

4. Solve: x - 7 = 2

5. Solve: x² - 5x + 6 = 0

Key takeaway 1. If you're trying to memorise steps, interleaving helps you learn to distinguish when to apply which procedure.

Key takeaway 2. If you're trying to understand the structural rule (e.g., all quadratics involve factoring or the quadratic formula), seeing grouped examples may help spot the pattern.

Example 2: Spanish Verb Conjugation

OK, so now let’s say you're teaching Spanish verb conjugation. Do you group all -ar verbs together before moving to -er and -ir (blocked practice)? Or do you mix them up from the start (interleaving)? The answer might be: both.

Each approach supports different types of learning. Blocking gives students a chance to notice the internal consistency within a verb group. They can see that all -ar verbs follow the same endings — o, as, a, amos, áis, an — without the distraction of competing patterns. Interleaving, on the other hand, mimics the real-world challenge of having to select the right conjugation rule based on a verb’s ending. It encourages learners to become fluent at spotting patterns and switching rules.

Blocked Practice: Practice all verbs of one type before moving to the next:

• All -ar verbs: hablar (hablo, hablas, habla...), caminar (camino, caminas, camina...)

• All -er verbs: comer (como, comes, come...), beber (bebo, bebes, bebe...)

• All -ir verbs: vivir (vivo, vives, vive...), escribir (escribo, escribes, escribe...)

Interleaved Practice: Mix conjugations across verb types:

• hablo, comes, vive, caminas, bebe, habla, escribo, comemos...

Who benefits from what:

• Memorizers benefit from interleaving because they practice the crucial skill of quickly identifying verb endings (-ar, -er, -ir) to apply the right conjugation pattern

• Rule-abstractors benefit from initial blocking because they can focus on how all -ar verbs share the same pattern (o, as, a, amos, áis, an) before dealing with variations across verb classes

Summary: When and How to Use Interleaving

The takeaway is not to use interleaving as an activity or strategy, but to be more precise about when and for whom it works and to view it as one lever in a broader ecosystem of learning. If the goal is to help students spot subtle differences (e.g., in art history or diagnosis), interleaving may help. But if they need to extract an underlying principle (e.g., grammar rules or physics laws), some initial blocking might serve them better.

Ideal Conditions:

• High similarity between rules: Use when spelling patterns are easily confused (e.g., "their/there/they're", silent letters, vowel patterns)

• Adequate prior knowledge: Students need foundational understanding before benefiting from interleaving

• Focus on discrimination: When learning goal is distinguishing between similar patterns

Avoid When:

• Introducing completely new concepts

• Working with struggling learners who lack basics

• Rules are highly dissimilar and unlikely to be confuse

For Students with Low Prior Knowledge:

• Begin with more blocked practice

• Provide additional scaffolding during interleaving

• Use visual supports and explicit feature highlighting

• Consider hybrid blocked-then-interleaved sequences

For Advanced Students:

• Increase complexity of interleaved patterns

• Include more subtle discriminative features

• Extend to morphological and etymological patterns

• Challenge with irregular exceptions to rules

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Further Reading:

Kornell, N., & Bjork, R. A. (2008). Learning concepts and categories: Is spacing the “enemy of induction”?Psychological Science, 19(6), 585–592. https://doi.org/10.1111/j.1467-9280.2008.02127.x

Firth, J., Rivers, I., & Boyle, J. (2021). A systematic review of interleaving as a concept learning strategy. Review of Education, 9(2), 642–684. DOI: 10.1002/rev3.3266

Little, J. L., Nepangue, J. A., & Longares, A. (2025). The optimal sequence for learning can depend on one's strategy: An individual differences approach. Learning and Individual Differences, 120, 102684. https://doi.org/10.1016/j.lindif.2025.102684