Re: The Base Rate Fallacy

From: HARNAD Stevan (
Date: Thu Jun 06 1996 - 22:45:42 BST

> From: "Hart, James" <>
> Date: Sun, 26 May 1996 21:59:40 GMT
> The Base Rate Fallacy states that people routinely ignore
> base rate frequencies and that it is an error to do so
> and base your conclusions instead on the similarity between
> an individuals personality and the prototypes of the
> categories under consideration.
> You will find in most experiments that base rates will be
> equated with prior probabilities. Subjects' judgements and
> their deviations between the Bayesian posterior probability
> are used to measure the extent to which the base rate
> fallacy has been committed.

Kid-sib doesn't know what Bayesian priors are -- not even what base
rates are...

> For example if there was a disease which takes two forms,
> both of which are fatal and require two different medicines,
> only one of which can be taken at any time. Form A occurs
> 10% of the time, form B 90% of the time. There is a test to
> see which type of the disease a patient has, this test is
> 80% reliable and it says that the patient has form A of the
> disease. The patient is likely to take the treatment for
> form A of the disease even though there is only a 10% chance
> that he has form A, and a 20% chance that the test was wrong
> and that he has form B. The patient is ignoring the base
> rate.

Still don't know what base rates are. To see what's needed for an A, see
the other replies on the thread for this topic.

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