Thursday, September 10, 2009

Economics and supermarket checkout lines

I remember reading a nice exposition of the efficient markets hypothesis on Andrew Leigh's blog a while back:
So if we believe shoppers are as rational as traders (why not?), then there should be a parallel efficient supermarkets hypothesis. The efficient supermarkets hypothesis tells us that: (a) the length of the line reflects all available information about its speed (cashier skill, size of preceding trolleys etc), (b) the best way of getting through the supermarket checkout is to pick the closest line and stay in it, and (c) the worst thing you can do at the supermarket is switch lines.

Of course, the number one problem with these things is the assumption that your fellow shoppers are rational in their choice of lines. Well, maths teach Dan Meyer has done the research at his local supermarket and it seems shoppers do not always rationally choose the shortest lines!
The express lane isn't faster. The manager backed me up on this one. You attract more people holding fewer total items, but as the data shows above, when you add one person to the line, you're adding 48 extra seconds to the line length (that's "tender time" added to "other time") without even considering the items in her cart. Meanwhile, an extra item only costs you an extra 2.8 seconds. Therefore, you'd rather add 17 more items to the line than one extra person! I can't believe I'm dropping exclamation points in an essay on grocery shopping but that's how this stuff makes me feel.

So there you go - watch out for the assumptions in your economic models!


Amber said...

I join the longest line so I can read the trashy magazines guilt free...

Your hypothesis doesn't take into account 'crazy'


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