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In this fourth edition of the CLT series, we're exploring the redundancy effect.

We've all done it at some point in our careers: the explanation is written on the slide, and we read it aloud anyway, just to be safe. Or, we add a paragraph under a clear diagram, in case anyone missed something. This often feels like extra support but for many students, it’s extra cognitive work.

Last week's edition was about putting things together. This one is about knowing when to take something away!

The redundancy effect is what happens when students are given information they don't need. Most often, that's the same information twice: words on a slide that we also say aloud, or a paragraph that describes a diagram students could already understand. Their working memory still has to process the extra version, and to check it against the first, so there's less left for learning (Sweller, Ayres & Kalyuga, 2011).

It sounds a lot like last week's split-attention effect, because both involve two sources of information. The difference comes down to one question:

A geometry statement like "angle ABC = angle XYZ" means nothing without the diagram, so it belongs on the diagram. A sentence saying "blood flows from the left ventricle into the aorta", under a diagram that already shows exactly that, adds nothing but extra reading.

In typical DistillED fashion, here's what the redundancy effect is, and what it isn't:

In one of the first studies on this, 30 Year 9 students learned how blood flows around the heart, lungs and body. Some studied a clear, labelled diagram on its own. Others studied the same diagram with a short explanation that described what it already showed, either underneath it or written onto it. The students with the diagram alone learned more, in far less time. Writing the explanation onto the diagram didn't rescue it (Chandler & Sweller, 1991).

That surprises most teachers, because the extra text was accurate and helpful-looking. It just wasn't needed, and reading it still cost something.

The most common version in classrooms is reading slides aloud. Ollie Lovell puts the problem simply: as teachers, we should only send one message at a time (Lovell, 2025). When we read out text that students are also reading, they're trying to follow two versions of the same words at slightly different speeds.

The research here is more nuanced than "never read your slides", which is useful news. Across 57 studies, adding the teacher's voice to text students were already reading didn't help on average. It did help students who were still learning to read fluently, and those reading in a second language. And slides showing a few key words worked better than slides with the full script (Adesope & Nesbit, 2012).

So the real question is whether students need both versions at the same moment. Usually they don't, and the fix is simple: say it or show it, or do one and then the other. The same logic applies to decorative extras: the clip art, background music or interesting-but-irrelevant story that sits alongside the learning. Students can't always ignore them, even when we'd like them to.

Here are three of the most common repeats in a classroom, and the fix for each.

The good news is that most redundancy can be fixed in a few minutes, usually by taking something away. The five steps below start with your resources and end with how you talk while students read.

The diagram below shows what this looks like on a single slide. Picture a typical one on the causes of the First World War: a title, a full paragraph explaining the alliance system, a stock photo of soldiers, and the teacher reading the paragraph aloud.

Now strip it back. Keep the title and a simple diagram of the two alliances. Add three key words: Triple Alliance, Triple Entente, obligation. Delete the photo. Explain the paragraph aloud while students look at the diagram, then give them a minute to write one sentence about why the alliances made war more likely.

The history is exactly the same. Students now have one thing to look at while they listen, and time to think afterwards.

👉 Read more on when information should be put together instead: The Split-Attention Effect

One last thing to keep in mind: redundancy depends on what students already know. An explanation that a novice needs can become redundant once students are more secure, which is the subject of a later edition in this series.

If you change one thing this week, take your next slide with a full paragraph on it. Cut it to key words, or give students time to read it before you speak. Just don't do both at once.

— Jamie

If you want more:

👉 Read The Split-Attention Effect: the other half of this decision, when to put information together

👉 Read Dual Coding: getting words and pictures to work together

👉 Read Element Interactivity: why extra load hurts most with complex content

👉 Read Cognitive Load Theory: the first edition in this series, and the big picture

👉 Get the Reducing Cognitive Load pack: practical ways to cut unnecessary load from your resources

👉 Get the Explanation and Clarity CPD Collection: for explaining clearly, with words and visuals working together

👉 Get the Memory and Cognitive Load CPD Collection: the fuller case for how working memory shapes learning

Cognitive Load Theory Unpacked One-Page Guide
To help you put this into practice, download Cognitive Load Theory Unpacked: a one-page guide to six strategies for managing what students must hold in mind, from checking prior knowledge to fading guidance as knowledge grows. Useful for your own planning, in coaching conversations, or during team PD.