Wednesday, 2 June 2010

Druids Demonstrate Regression to the Mean perfectly

I once heard of an experiment that a friend of mine (an anti-speed camera campaigner) used to do when he was giving a presentation. He would get everyone in the room to randomly generate a 2-digit number (not sure how, exactly, maybe he used to carry some sort of 10-sided dice with him). He would then give everyone who rolled more than 80 a big picture of a speed camera and got them to generate a second set of numbers. Lo and behold, only a small fraction (on average around 1/5) of those carrying speed camera signs had high levels of deaths. The speed cameras had worked!

This is the purest way I've ever seen to explain the phenomenon of 'regression to the mean'. It's a well-known phenomenon, and explains a lot of things, from why test scores at the worst schools tend to improve the next year, through why patients who visit a homeopath seem to feel better, to the Sports Illustrated Jinx.

However, the Austrian motorway authority seems never to have heard of it. They recently asked some druids to reduce the number of fatalities at a few accident blackspots by burying some magnetic slates. The results were a roaring success:
Austrian motorway authority ASFINAG said it was sceptical at first and kept the project a secret. But it went public after the druids’ efforts cut the number of deaths at the notorious crash site from six a year to zero in two years.
I don't think there's much more to say.

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