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

[Peter Dalgaard]

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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