Interleaving — Why Mixing Topics Works Better

When you sit down to revise, the natural instinct is to focus on one topic until you feel you have mastered it before moving on to the next. You study photosynthesis until it feels solid, then move on to respiration, then to ecology. This approach is called blocked practice, and it is how the vast majority of students organise their revision.

It feels logical. It feels thorough. And it is significantly less effective than the alternative.

What Is Interleaving?

Interleaving is the practice of mixing different topics, problem types, or subjects within a single study session rather than studying them in separate blocks. Instead of spending an hour on photosynthesis followed by an hour on respiration, you alternate between the two — perhaps spending 15 minutes on photosynthesis, then 15 minutes on respiration, then back to photosynthesis, then on to ecology.

Blocked Practice	Interleaved Practice
AAAA BBBB CCCC	ABCA BCAB CABC
Study photosynthesis for 60 min, then respiration for 60 min	Alternate between photosynthesis, respiration, and ecology in 15-min blocks
Feels smooth and easy	Feels choppy and harder
Better performance during the study session	Better performance on the actual exam

The last row is the key. Blocked practice produces better results during practice but worse results on the test. Interleaving produces worse results during practice but dramatically better results on the test.

The Research Behind Interleaving

The Rohrer and Taylor (2007) Study

Students learned to calculate the volumes of four different geometric shapes. Half the students practised in blocks (all problems of type A, then all of type B, etc.), while the other half practised in an interleaved order.

During practice, the blocked group performed better — they got more problems right because each problem was the same type as the one before. But on a test one week later, the interleaved group scored 43% higher than the blocked group.

The Kornell and Bjork (2008) Study

Students learned to identify the painting styles of different artists. Those who studied the painters in an interleaved fashion — seeing paintings by different artists mixed together — were significantly better at identifying new paintings by those artists than students who studied each artist's work in a block.

This study is particularly important because it shows that interleaving does not just help with maths problems — it improves the ability to discriminate between categories, which is relevant to almost every subject.

Why Interleaving Works

flowchart TD
    A[Interleaved practice] --> B[Forces you to identify<br/>WHICH strategy to use]
    A --> C[Forces you to discriminate<br/>between similar concepts]
    A --> D[Strengthens retrieval<br/>by adding context switching]
    B --> E[Builds the selection skill<br/>that exams actually test]
    C --> F[Reduces confusion between<br/>similar topics on the exam]
    D --> G[Each retrieval is harder<br/>= stronger memory]
    E --> H[Better exam performance]
    F --> H
    G --> H

Interleaving works through three main mechanisms:

1. Discrimination Learning

When you study photosynthesis in a block, every problem is about photosynthesis. You do not need to figure out what type of problem it is — you already know. But in an exam, no one tells you which topic a question belongs to. You have to identify the topic, select the right approach, and then apply it.

Interleaving forces you to practise this discrimination during revision. By mixing photosynthesis, respiration, and ecology problems, you must first work out what the question is about before you can answer it. This mirrors the actual demands of an exam.

2. Contextual Interference

Switching between topics creates what researchers call contextual interference — the cognitive friction of shifting between different sets of knowledge and strategies. This friction is a desirable difficulty. It slows you down during practice but strengthens the underlying learning.