[R] Fw: Logistic regresion - Interpreting (SENS) and (SPEC)

Frank E Harrell Jr f.harrell at vanderbilt.edu
Tue Oct 14 05:47:28 CEST 2008

Gabor Grothendieck wrote:
> On Mon, Oct 13, 2008 at 11:21 PM, Frank E Harrell Jr
> <f.harrell at vanderbilt.edu> wrote:
>> cryan at binghamton.edu 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.


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 E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University

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