Three views of a statistics puzzle

3 thoughts on “Three views of a statistics puzzle”

  1. So for me, at first, this seems to yield a counterintuitive result: the positive predictive value of independent tests goes up as the incidence goes down!

    But this an artifact of the specific example artificially fixing the posterior probabilities; for any given fixed PPV, as the incidence rises the test characteristics have to get worse (either SN or SP or both) to keep the same PPV.

    I could see where this would trip most people up, as it did me at first, because having a fixed PPV is such a weird and un-realistic phenomenon.

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