Venn and the Art of Categorisation Maintenance

This post is based on my presentation at the Durrington High School Teach Meet #DHSTM16. It was the best TeachMeet that I’ve attended so far. Massive thanks to @shaun_allison and everyone else involved. Do follow the DHS Class Teaching blog.

Although I now work on the PGCE courses at the University of Southampton, I used to be a proper teacher, and this is the TRUE story of how I learned to teach the photoelectric effect.

It doesn’t matter if you’ve never heard of the photoelectric effect. Einstein won the Nobel Prize for Physics for his work on this, and it represents a crucial step in the development of quantum mechanics – trivial stuff – but it is only a very small part of the A-Level spec. It might be helpful to know that, at it’s core, is a simple conservation of energy concept (KS3) but the photoelectric effect sets this in an unfamiliar and complicated sub-atomic context.

The first time I came to teach it, I knew it might be hard. Reviews of cognitive psychology suggest there might be about six techniques that pretty consistently improve the effectiveness of teaching and learning. One of them is pairing of words with graphics to support explanations.

The idea is to provide students with graphics and then link oral explanations closely to these. It seems to be important to keep the graphics as simple diagrams without lots of lebelling or other text. So that’s what I did; it was a great explanation! Then I added complexity with a practical demo. Then assessment. And they hadn’t a clue.

I did my best to repair the damage, went back over the explanation, asked scaffolded questions, modelled exam questions. It wasn’t awful but my experience is that, once students think something is difficult to understand, it’s very hard to come back from there.

The second time I came to teach it, I knew that pairing words with graphics wasn’t enough. Reviews of cognitive psychology suggest there might be about six techniques that pretty consistently improve the effectiveness of teaching and learning. Another of them is that abstract concepts should be linked to concrete representations. Perhaps I needed to do that more effectively. After some hard thinking I came up with a great analogy based on a coconut shie. This activity took about five minutes for the students to do, and another five to review with them, and then came my killer explanation (pairing words with graphics), some complexitiy was added with a practical demo. Then assessment. And this time they all understood.

So we moved on to do some past paper questions and suddenly they were all at sea again. Despite taking a step back and modelling some of the exam questions for them, I couldn’t completely shake off a feeling in the class that the photoelectric effect was just too hard. It was very frustrating.

The third time I came to teach it, I knew that linking abstract concepts to concrete representations, and pairing words with graphics helped a lot with understanding but there was still a problem. I reviewed the past paper questions and started to wonder whether the rather odd term ‘photoelectron’ might be causing difficulty. Because I thought they were clear about what an electron was, and what a photon was, they would have been fine with me telling them that a photoelectron was just an electron. But what if they weren’t completely happy with electrons and photons?

So I tried this, giving the students a list of features to add to the diagram.

venn electrons photons

And, at the third time of asking, I finally nailed it! This time they not only got it but could handle past paper questions; they didn’t even find it hard.

I’ve used Venn diagrams as a categorisation exercise a lot since then and found them very valuable. They’re really good for when things are similar or related but with important differences. I think, as a teacher, it’s easy to assume that the distinctions are clear when, for students who have less certain concepts, actually the overlap is a major source of confusion. The Venn diagram forces them to focus on the similarities and differences; it makes them think hard about these; and possibly it reduces cognitive load because there are no other distractions.

I usually give students the points to go on the diagram (on the board or a slide, although you can cut and stick if that floats your boat) but for revision, or possibly in a few cases if you are setting up a debate, this can be left open.

You can also do diagrams with three sets but my experience is that this only works if it’s fairly straightforward or familiar.

The other categorisation exercise I use a lot I don’t have a name for but looks like this:


Students draw this on a sheet of paper and then have to put crucial factors in the middle, moderately important ones in the outer ring, and unimportant ones outside. It works for the same reason as the Venn diagram but this time it is about relative importance rather than similarities and differences.

If you haven’t already tried this kind of categorisation exercise. I recommend having a go. Sir Tim Brighouse was talking at the beginning of the TeachMeet about low effort, high impact, and I think this fits the bill.

It is also easy to ask probing questions around these exercises. “Why did you put X there?” is all that’s required. And that takes me back to those six or so techniques that pretty consistently seem to improve the effectiveness of teaching and learning. I guess you might want the full list.

  • Pairing words with graphics
  • Linking abstract concepts with concrete representations
  • Posing probing questions
  • Repeatedly alternating worked examples and problems that students must solve
  • Retrieval practice
  • Distributing practice

The review comes via the What Works Clearinghouse at the Institute of Education Sciences – a branch of the US Department of Education. It reviews evidence on the effectiveness of interventions (similar to what the EEF does in the UK) using a very high minimum standard of evidence. This list has been around for nearly ten years but several items haven’t been picked up elsewhere until pretty recently; perhaps it should have been.

Best wishes