Gabor Grothendieck wrote:
On Mon, Oct 13, 2008 at 11:21 PM, Frank E Harrell Jr
<[EMAIL PROTECTED]> wrote:
[EMAIL PROTECTED] wrote:
I recall a concept of Snout: sensitivity that is high enough to
essentially rule out the presence of disease. And Spin: specificity that
is high enough to essentially rule in the presence of disease.
So perhaps the below is backwards? The higher the sensitivity, the
greater the NPV? And the higher the specificity, the
greater the PPV?
Why should we care when we can directly estimate Prob(disease | test results
and risk factors)?
Sensitivity and specificity are functions of the test only but ppv is
also a function
of the disease prevalence. Just change the prevalence and the ppv changes
whereas sensitivity and specificity are invariant.
Gabor,
That's a very common belief but it turns out not to be true. See
references from my earlier post. Sensitivity and specificity are only
invariant in you don't analyze how they vary.
Also, much research does not understand what prevalence really means.
It actually could be argued to not be a scientific quantity as its
meaning depends on unspecified mixtures of subjects.
If our aim is to assess a test one wants a measure that only measures the test
itself.
There is no such measure. The performance of a test depends on the type
of patient being tested as well as other things.
Frank
--
Frank E Harrell Jr Professor and Chair School of Medicine
Department of Biostatistics Vanderbilt University
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