The tricks that supermarkets play to stop you from comparing on price

So here I am in my local Sainsbury’s, doing my weekly shopping. Let’s see, what’s first on my list…? Ah, Apples. I like buying organic, if it’s not too much more expensive. Lets look at the prices:


A bag of organic apples is £2.29, and a bag of normal ones is £1.89. OK the organic ones aren’t much more expensive, I’ll take them.

… But hang on a minute! If I was to take a closer look, I’d see the organic bag has 5 apples in, and the normal bag has 6. If I looked even closer I’d see that the price per apple is shown. That’s nice – it allows me to make an instant price comparison – if, that is, I am willing to take the time to lean in and read the small print. Since 30-40% of people in Europe and the US have myopia of some form, leaning in can mean having to get really close. And since 100% of us are in a hurry when we do our shopping, having to stop to read small print is frustratingly time consuming. It’s much easier to just make a quick judgment call based on your gut feeling. This is exactly what the supermarkets want you to do – inaccurate estimations and emotional judgments.

When you have about 800 stores and 16 million customers per week, persuading a small fraction more customers to buy the higher price items can equate to a healthy increase in profit. In this case, a bit of of smoke and mirrors involving typography and bag size is probably just enough to produce the results they want.

Let’s see what’s next on my shopping list… Ah, aubergines. Do I buy the bag of “basics” aubergines, or do I buy the much juicer looking individual ones?


I know from earlier that I can just read the small print to make a direct comparison. …Er, wait a minute. Where’s the helpful small print? It’s gone.

Now, if I knew the weight of each item, I could whip out my trusty pocket calculator and do an exact comparison. But since the unpackaged aubergines are priced per item, and the packaged ones are priced by weight, I can’t work it out. Do you remember how, when you were a kid, you used to see scales in the fruit and vegetable aisle? Funnily enough, you don’t get them any more. Since I can’t compare on price, I am going to have to make an emotional judgment based on attractiveness. Mmmm, that individual one sure looks beautiful. Anyway, lets continue with my shopping. Next on the list is onions.


Look, organic onions are only 1 penny more. What a bargain! … No, wait, the organic bag is 750g, and the normal bag is 1kg.

Once again they are messing with the bag sizes to reduce the perceived price difference. If the organic bag was also 1kg, it would be £1.28. Perhaps this is a big enough difference to make a percentage of their 16 million customers per week to opt for the cheaper option. Meaning less revenue. Hmmm.

Estimating 25% of 96p isn’t particularly hard, but it’s hard enough. According to Gross-Tsur et al. (1996), 6.5% of the population suffer from dyscalculia, (the math equivalent of dyslexia) making this kind of mental arithmetic tricky for them. And in any case, even if you’re pretty good with your mental arithmetic, you don’t want to be having to do hundreds of calculations every time you go to the supermarket. Doing your weekly shopping takes long enough as it is.

The clever persuasive psychological techniques used by supermarkets is something that’s written about in Tim Harford’s Undercover Economist, and Paco Underhill’s Why We Buy. If you’re interested, go read them.