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

John Sorkin jsorkin at grecc.umaryland.edu
Tue Oct 14 00:39:47 CEST 2008

```Indeed, however as I stated in my prior Email, the cases of a 0 or 1
prevalence are degenerative and are of little practical importance. And
as noted in my EMail message, I was talking about values of PPV and NPV
as a function of sensitivity and specificity when the prevalence is
fixed.
John

John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)

>>> Peter Dalgaard <p.dalgaard at biostat.ku.dk> 10/13/2008 6:28 PM >>>
John Sorkin wrote:
> Of course Prof Baer is correct the positive predictive value (PPV)
> and the negative predictive values (NPV) serve the function of
> providing conditional post-test probabilities PPV: Post-test
> probability of disease given a positive test NPV: Post-test
> probability of no disease given a negative test.
>
> Further, PPV is a function of sensitivity (for a given specificity
in
> a population with a given disease prevalence), the higher the
> sensitivity almost always the greater the PPV (it can by unchanged,
> but I don't believe it can be lower) and as NPV is a function of
> specificity (for a given sensitivity in a population with a given
> disease prevelance), the higher the specificity almost always the
> greater the NPV (it can by unchanged, but I don't believe it can be
> lower) .
>

The PPV and NPV can be anything between 0 and 1 regardless of
sensitivity and specificity. Just apply the test to populations with a
prevalence of 0 or 1. The former gives you a PPV of 0 and an NPV of 1
since none of the positive and none of the negative will be true
positive. And vice versa.

--
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