Go placidly amid the noise and haste, and remember what peace there may be in silence.

As if you couldn’t tell that I’m putting off doing the analysis problem sheet for tomorrow morning…

Many people believe that if they find a good deal in a supermarket, then it is them who are at an advantage – while some may argue “you’re more likely to buy from them in the future”, “you’ll get hooked on it” or similar, people generally believe that if you take advantage of an offer when it is on, and cease your purchases when it stops, then you’ll ultimately end up paying less for more. Economists call it an increase in consumer surplus.

However, in a maths lecture last week when I didn’t have anything better to do, I started thinking. I used an example – at the local Sainsbury’s, they had just stopped a special offer whereby you could get 10 cans of Coke or Diet Coke for £3.80. With each can containing 330ml of the fizzy liquid, this means that you’re getting 3.3 litres for £3.80, or £1.15/litre. Comparing this to normal priced cans, which typically go for £0.65, this is approximately equal to £1.97/litre, or if you’re partial to the 500ml bottled variety, it works out at approximately £2/litre. So the original deal is a good deal, is it not? You’re getting more Cokey-goodness for your money.

From an economics standpoint this may be all well and good. But then think about the psychological aspect of things. Yes, maths and psychology. What a mix. Imagine buying one of these 10-can monstrosities, which you then proceed to take home and stack in your fridge. Compare this to having only, say, two cans of Coke in your fridge. You are much more likely to consume more Coke when you have a huge stack of them in reserve, even if you aren’t particularly thirsty – in fact, you’ll probably start pulling one out of your fridge every now and again due to habit more than anything else. So your consumption of your Coke in your fridge will increase with the heightened number of cans of coke in the fridge. But you’re still getting good value for money, right?

Let’s continue the example from earlier. Say you consume an average of 4 cans of Coke every week. (Yes, a very light Coke drinker, but it makes the maths easier.) These 4 cans will cost you £2.60 on individually priced cans, or £1.52 if they are a part of the 10-pack described above. Now let’s assume that your purchase of the 10-pack increased your consumption to 7 cans a week, or one a day. That brings the total cost to £2.66, which is more than what you’d pay for the individual cans. Yes, you’re getting a lot more Coke for your money, but you’ll be consuming it a lot more often. It’ll run out a lot faster too – whereas with 4 cans the multipack would last you 2 1/2 weeks, the 7 cans will last you under 1 1/2. So you’ll need to buy more cans more frequently. Loss of money will follow.

Alright, you say, but an increase from 4 cans to 7 cans is an increase of more than 50% – someone wouldn’t alter their Coke-drinking habits that much, would they? That’s where the psychological element comes in. You want to get your value for money, so you drink a lot more. It can also become a habit, meaning that you’ll keep drinking it even when you don’t particularly need to. And when the supermarket stop the offer (as they will ultimately do), your body will be screaming out for it’s daily dose of Coke, and you’ll be forced to pay full price for your individual cans. (Note as mentioned above that cans provide better value for money than bottles.)

Finally, the last reason big multipacks of Coke or biscuits or chocolate or sweets are evil – they’ll make you fat. And nobody wants that.

- David