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Monday, April 03, 2006

What is probability?

We’re having a small disagreement here concerning just what probability is. One colleague claims that it’s an indication of belief: if I say that there’s a 30% probability of rain, then that means that I believe that there are about three chances in ten that it will rain.

I contend that probability is empirical. If it rains on 30% of the days on which I had predicted a 30% chance of rain, then my predictions were pretty good. Belief has nothing to do with it. If it turns out that it rains on half of such days, then my statement was incorrect, but that’s no different from someone who says that the sine of 30° is 2. Belief doesn’t enter into it; both statements were simply wrong.

Presumably we’re not talking here about straightforward theoretical probability, as when we say that the chances of rolling a sum of three on two standard dice is two out of 36.



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