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Old 07 February 2014, 08:14 PM
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Richard W Richard W is online now
 
Join Date: 19 February 2000
Location: High Wycombe, UK
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I once had a long argument with my dad about that one, Steve. I think Ben Goldacre (Bad Science guy) used it to illustrate a statistical point in one of his articles, and my dad was convinced he was wrong - which he thought was a shame because he also likes Ben Goldacre. Usually my dad is good at this stuff, but in this case he just couldn't see it.

I wasn't able to convince him of the right answer but I thought of a good way to do so later - although I've not tried it on him because I didn't have an opportunity for ages and I'm sure he's forgotten the conversation by now.

Instead of a test for a disease, imagine it's a unicorn detector - and simplify the odds a bit. If you point it at a real live unicorn, it's 100% accurate and identifies it every time. But if you point it at any other object, it's only 99% accurate and gives a false positive 1% of the time. If you point it at a random object and it tells you the object is a unicorn, what's the chance that it actually is a unicorn?

(Strictly speaking there's not enough information in my version to answer, but I'm assuming that people will see the point that real live unicorns don't exist. No linking to photos of rhinos or surgically-altered goats; those wouldn't fool my device.)
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