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I'm just saying, 90% true positive rate, 10% base rate, 1000 pts:

* 100 true positives

* 100 false positives (100-90=10% of 1000)

That's still just a 10% false positive rate across the population, but if you get a positive test, it's only 50/50 correct.

Did I do this wrong? I'm assuming I did this wrong.

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I think you may be close but likely for the wrong reasons. I had to sit down with this for a moment to feel comfortable with it.

If we take your 10% base rate to be disease prevalence, that gives us 100 sick and 900 well.

Of the 100 sick, something with 90% sensitivity should get me 90 true positive tests and 10 false negative tests.

Of the 900 well, I should expect to see for a test with 90% specificity, what, 80 false positives and 810 true negatives? if I did my arithmetic right?

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