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 <[EMAIL PROTECTED]> 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. -- O__ ---- Peter Dalgaard Ă˜ster Farimagsgade 5, Entr.B c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 Confidentiality Statement: This email message, including any attachments, is for th...{{dropped:6}} ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.