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From |
"Lachenbruch, Peter" <Peter.Lachenbruch@oregonstate.edu> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: AW: ppv and npv |

Date |
Mon, 9 Nov 2009 08:57:02 -0800 |

This is a handy ado file. There is one issue that I have with NPV and PPV. In most studies to determine sensitivity and specificity, the samples are conditional on disease status, so the prevalence of the disease in the training sample is quite different from what holds in most applications. For example, in a AIDS clinic, the prevalence of disease is likely to be at most 0.05, while the training sample may be 50% HIV positive. The PPV would then be p*Sensitivity/(p*Sensitivity+(1-p)(1-Specificity)) where p is the prevalence of the HIV. If Sensitivity and Specificity are 0.95 (actually a bit low for HIV), if we assume prevalence is 0.5 we would have "PPV"=0.5*0.95/(0.5*0.95+0.5*0.05)=0.95 If we use a prevalence of 0.05, we would have 0.05*0.95/(0.05*0.95+0.95*0.05)=0.50 In a blood donation center, the prevalence is more like 0.01, the PPV would be 0.161 It is usually a good idea to compute PPV and NPV for a variety of prevalences as a test may be used in a number of different contexts. Tony Peter A. Lachenbruch Department of Public Health Oregon State University Corvallis, OR 97330 Phone: 541-737-3832 FAX: 541-737-4001 -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Martin Weiss Sent: Monday, November 09, 2009 4:35 AM To: statalist@hsphsun2.harvard.edu Subject: st: AW: ppv and npv <> Try Roger`s -ssc d senspec- HTH Martin -----Ursprüngliche Nachricht----- Von: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Celine Genty Gesendet: Montag, 9. November 2009 13:16 An: statalist@hsphsun2.harvard.edu Betreff: st: ppv and npv Hello, The "roctab" command give for each cutpoint sensitivity / specificity / LR+ / LR-. I need PPV and NPV for each cutpoint too... How can I do ? Thank you. Céline Genty * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: ppv and npv***From:*Celine Genty <Celine.Genty@imag.fr>

**st: AW: ppv and npv***From:*"Martin Weiss" <martin.weiss1@gmx.de>

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