re: "Half the schools are below average" - not always true

quote:

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Wouldn't the answer be 99%?

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Nope.

Its a LOT less. 1 in 11.

Proof:

1% false positive means that out of a population, of say, 1000 - 10 people will test false positive.

But since the known disease rate is 0.1% - only one person out of that population of 1000 will actually have the disease.

So there are 10 people who test false positive for every one person who actually has the disease.



(I forgot to mention there are no false negatives in this example)