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
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