Sum versus Average as a persuasion tool

Adverts regularly appear on TV offering a book plus toy car, or a book plus model of the solar system, for a staggering price around three hundred pounds. How do they get away with it? Well, they don’t describe it as I have. Instead of a book, you are getting dozens of issues of a weekly magazine which has one component of the toy or model free with each issue.

Daniel Kahneman’s Nobel prize lecture makes that point that some quantities are much more perceptually available than others. When you see a crowd of people standing around a building, you immediately have a sense of the crowd’s average height; whether it’s a group of tall, average or short people. You don’t perceive the sum of their heights  with the same immediacy. Is the total height of the people greater or less than that of the building? It might take some concentration to be sure of an answer to that. Of these two objective quantities – the sum and the average – one needs deliberate mental arithmetic to estimate and the other is estimated automatically by your perceptual system.

Since the average is more psychologically available, we can potentially be biased towards the average when we should be calculating the sum (an example of an anchoring effect). This can be used as a selling technique. TV adverts promise that you can build a brass model of the solar system, a wooden model ship or a collection of figures from parts given away each week with a series of magazines. The first magazine is just three or four pounds and the normal price (in smaller type) is six pounds. Buy an issue a week for a year and you’ll have the advertised object.

Phrasing it this way gets the consumer to focus on the small numbers – four pounds; six pounds – rather than the actual cost, which is around 300 pounds. Unfortunately, the customer’s bank balance will go down by the sum of the payments, not the average.

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