Barnum Effect

James Randi has another applicant for his million dollar 'paranormal challenge' prize (see SHARE MY BURDEN in his latest newsletter); although dressed up as astrology, the scam that this particular chap is using is based on the 'Barnum Effect' for which there is a very good definition (and example) in the Skeptic's Dictionary

Derren Brown did this on one of his shows; he had three groups of people (in Spain, USA and UK, I think), none of whom he knew anything about (although they were all fairly young); he gave each person a 'personal' description of themselves running to several pages; they all marked their personal descriptions as being highly accurate; he then asked them to randomly pass their description around the group, if they ended up with the same one back they should pass it on - of course, they just kept passing them round because they were all identical.

Randi's problem (apart from having what we Brits would think of as a bit of an 'ooh er, missus!' surname) is defining an appropriate protocol for testing the applicant's claim. The outcome should be random provided he doesn't meet the victims beforehand (or their wives) but just gets their astrological details (so he can't embroider the horoscope with observed information) and the victims look at all the descriptions (and in random order) before choosing which one is the best fit.

I suspect this guy knows exactly what kind of scam he's running and will run a mile as soon as he sees any kind of real test, but if he really believes in what he does then the problem will be preventing him from arguing the toss after he's failed e.g. by suddenly claiming that you need to know the exact time of birth as well as the date or by claiming that the test merely shows that more than one horoscope was a good fit rather than that he was wrong (and all his descriptions will be good fits as per the Barnum Effect).

I'm going on a bit now, perhaps I should stop.