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I'm not sure talking about groups/populations is helpful.
You're probably right, but because I misunderstood the point from the start (I think!), it was necessary to distinguish between the calculated sensitivity/specificity and the actual test results.
You just test an individual and there are 2 possibilities: a positive result or a negative. Of the positives approx. 50% are false results, which is a failure of the test. Of the negatives, 99% are accurate and 1% are false negatives. It is much more likely that the testee is negative so the false positives are fairly common and false negatives are an (unlikely) test failure on top of an (unlikely) result, meaning a very small proportion. Hence, negative results are more likely to be true than positive results.
I agree with this (and Greenbank's many examples of it using actual numbers). There is zero that is contentious about this.
You are correct that the sensitivity and specificity never change. Just the likelihood of the result being true or false.
This is what I assumed, but because of the initial confusion, I thought there may be something happening in medicine which I didn't understand or know about. Hence my original confusion/question.
I don't think you actually disagree with each other
Ha, all I can say is that I don't disagree with your take on it.
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I'm not sure talking about groups/populations is helpful. You just test an individual and there are 2 possibilities: a positive result or a negative. Of the positives approx. 50% are false results, which is a failure of the test. Of the negatives, 99% are accurate and 1% are false negatives. It is much more likely that the testee is negative so the false positives are fairly common and false negatives are an (unlikely) test failure on top of an (unlikely) result, meaning a very small proportion. Hence, negative results are more likely to be true than positive results.
You are correct that the sensitivity and specificity never change. Just the likelihood of the result being true or false, which is what @Greenbank meant by "66% accurate" or whatever (i.e. 66% of these are true, the rest are test failures)
I don't think you actually disagree with each other