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Ah, brilliant, makes perfect sense put that way. Thank goodness they're not just teaching you communication skills at med school these days! Regards AF ----- Original Message ----- From: "Michael Stewart" <[log in to unmask]> To: <[log in to unmask]> Sent: Thursday, March 02, 2006 7:51 AM Subject: Re: SAH >> My rudimentary stats tells me that PPV is closely related to specificity, >> and NPV is closely related to sensitivity, so is it really correct for >> you to say sens and spec are useless at interpreting results of a test? >> In other words, a highly sensitive test (with few false negatives) will >> also produce a high NPV, while a highly specific test (with few false >> positives) will also produce a high PPV, won't it? Am I missing something >> here? > > While I am (very definately) not a statistician, hopefully I can put this > sensibly. > > Sensitivity and specificity are technical parameters of a test, in > isolation from any particular population. If you feed it a definate > positive (sens) or negative (spec), they indicate the odds of it giving > you the correct answer. > > PPV/NPV relate to the application of the test to a particular population, > and depend on the prevalence of the condition as well as the test > parameters. Consider a test with a very high sens/spec (but still less > than 100%), and use it on a large population entirely free from the > disease in question. There will be a few false positives, no true > positives, and the PPV will be 0%. As the condition becomes more prevalent > in the population, the number of true positives increases and false > positives decreases, and the PPV rises - despite the test being unchanged. > A similar (reverse) trend happens for NPV - the less prevalent the > condition, the higher the NPV for a given test. > > I have no idea how this will survive e-mail formatting, but worth a try: > > Population > T With disease Without disease > E Positive a b > S Negative c d > T > > Sens = a/(a+c) > Spec = d/(b+d) > PPV = a/(a+b) > NPV = d/(c+d) > > Regards > > Michael > -- > Medical Student