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xv15 comments on MetaMed: Evidence-Based Healthcare - Less Wrong
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"False positive rate" and "False negative rate" have strict definitions and presumably it is standard to report these numbers as an outcome of clinical trials. Could we similarly define a rigid term to describe the probability of having a disorder given a positive test result, and require that to be reported right along with false positive rates?
Seems worth an honest try, though it might be too hard to define it in such a way as to forestall weaseling.
If I understand the following Wikipedia page correctly:
http://en.wikipedia.org/wiki/Positive_predictive_value
The term you are requesting is Positive predictive value and Negative predictive value is the term for not having a disorder given a negative test result.
It also points out that these are not solely dependent on the test, and also require a prevalence percentage.
But that being said, you could require each test to be reported with multiple different prevalence percentages:
For instance, using the above example of Downs Syndrome, you could report the results by using the prevalence of Downs Syndrome at several different given maternal ages. (Since prevalence of Down's Syndrome is significantly related to maternal age.)
thanks, PPV is exactly what I'm after.
The alternative to giving a doctor positive & negative predictive values for each maternal age is to give false positive & negative rates for the test plus the prevalence rate for each maternal age. Not much difference in terms of the information load.
One concern I didn't consider before is that many doctors would probably resist reporting PPV's to their patients because they are currently recommending tests that, if they actually admitted the PPV's, would look ridiculous! (e.g. breast cancer screening).