Supermarket queues

21st February, 2004

Dear Economist,

How do I choose the shortest queue at the supermarket?

Yours sincerely,

P.N., Aylesbury

Dear P.N.,

Mathematicians reckon the odds are against you. If you choose a queue at random, there will be a line on either side of you, and thus a two-thirds chance that one will be faster.

Economists take a more sophisticated view. David Friedman, for instance, argues that the relevant discipline is financial market theory. Choosing the right queue is like picking the right portfolio of shares: if it were obvious which shares were good value, they wouldn’t be good value any more. If it were obvious which queue would be quickest, everyone would join it. Naive attempts to “beat the market” will fail.

Then there is “efficient market” theory – you can’t out-perform a random choice of shares because public information is immediately incorporated into share prices. In truth, most markets are not efficient and thus it is possible for an informed decision-maker to beat them. Even if supermarket queues were efficient, no queue would be a superior bet, because expert supermarket customers would quickly join any queue that was likely to be quicker.

More likely, queues are not efficient because few have much to gain from becoming expert queuers. Some have other considerations, such as minimising the distance walked, while others shop rarely, so the calculations are more trouble than they are worth.

And unlike the stock market, which a financial wizard can make more efficient by outweighing the foolish decisions of small traders, in the supermarket a single expert queuer has a limited effect on the distribution of queuing times.

I can advise you to steer clear of elderly ladies with vouchers, but more advice would be self-defeating. Too many of your rivals would read it.

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