Interleaving: a short guide

How strategic variety of boundary conditions enhances learning

Feb 22, 2025

Core Principles: Getting the Basics Right

What is Effective Interleaving?

Interleaving is a learning strategy that involves mixing different topics or tasks that require related knowledge and skills, rather than studying them in blocks.
The aim of interleaving is to help learners compare and discriminate between related items.
Interleaving can happen during a single lesson or across multiple lessons

Why is it effective?

Interleaving essentially combines the benefits of both the spacing and testing effects. The spacing effect is the gap between the initial study and subsequent review of a concept, which causes forgetting and requires retrieval while the testing effect strengthens recall through the act of retrieving information

But effective interleaving isn't about random mixing, it's about creating meaningful connections. When done right, it helps students develop more a robust understanding of a domain of knowledge. For example, in a math class, instead of drilling one type of problem repeatedly, you might present a thoughtful mix of related problems that require different approaches but share underlying principles.

Key point 1. Interleaving is about boundary conditions

The effectiveness of interleaving isn't universal but operates within specific boundary conditions. Research shows that interleaving works best when the concepts being mixed are related enough to form meaningful connections, yet distinct enough to require active discrimination between them. For example, interleaving practice with different geometric shapes works well because students must notice crucial differences while recognising underlying mathematical principles. However, as task complexity increases, there's a sweet spot; benefits grow up to a certain point before potentially overwhelming students' cognitive capacity. This relationship follows an inverted U-curve, where too simple or too complex tasks diminish the advantages of interleaving.

Perhaps most crucially, a student's prior knowledge plays a vital moderating role. Strong foundational understanding allows students to handle the additional cognitive demands of interleaved practice, while insufficient background knowledge can turn productive struggle into unproductive confusion. This explains why interleaving should typically be introduced after basic mastery is achieved, not during initial learning of concepts.

In our new online course, we created examples and non-examples of effective practice and looked specifically at lethal mutations of the science of learning. We believe it’s not just enough to know how something works, but also how it doesn’t work. Here’s one example:

Key point 2. Interleaving works after foundational Knowledge is Secure

Interleaving works best after students have gained some level of competency. Trying to interleave complex material too soon—before learners have mastered the basics—can create frustration and hinder progress.

What goes wrong?

Students lack the foundational knowledge needed to make sense of interleaved material.
Instead of strengthening learning, they feel lost and disengaged.

How to get it right:

First, ensure strong foundational knowledge through direct instruction and guided practice.
Gradually introduce interleaving once students demonstrate some mastery—it should be a challenge, not an overwhelming struggle.

Bjork & Bjork's work (2011) placed interleaving within their broader "desirable difficulties" framework, showing how strategies that make learning temporarily harder can enhance long-term retention. Here is Robert Bjork explaining this:

Key point 3. It works up to a point

At moderate levels of complexity, it enhances learning by encouraging discrimination and deeper processing, but beyond a certain threshold, the cognitive demands become overwhelming. Initially, interleaving helps learners by forcing them to retrieve and apply knowledge flexibly, strengthening their ability to distinguish between concepts.

However, as task complexity increases, either due to insufficient prior knowledge, too many interleaved topics, or excessive switching, the benefits diminish. At this point, the additional cognitive load impedes learning rather than supporting it, leading to frustration, surface-level understanding, or disengagement. This highlights the importance of calibrating interleaving to the learner's expertise; it should challenge but not overload.

Common Pitfalls to Avoid

✔️ Mix related topics within the same domain.
✔️ Integrate and connect concepts thoughtfully.
✔️ Use interleaving after foundational knowledge is secure.

❌ Randomly alternating between completely different subjects.
❌ Jumping too quickly between unrelated topics.
❌ Interleaving before students understand the basics.