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Re: st: sensitivity, specifity and missing data


From   Philippe Glaziou <glaziou@pasteur-kh.org>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: sensitivity, specifity and missing data
Date   Tue, 4 Feb 2003 11:40:23 +0700

BISSERY Alvine <Alvine.BISSERY@hop.egp.ap-hop-paris.fr> wrote:
> I want to estimate the specifity of a test. Response of the
> test are : "disease", "not disease", and "i don't know" The
> gold standart gives : "disease" , "not disease".  What do I
> have to do with the answer "I don't know" .. how can I estimate
> the specificity ?


You can compute the estimate the usual way: the specificity is
the conditional probability that the test will be negative given
a negative result of the gold standard. 

Therefore, the specificity equals the number of samples with a
negative test result divided by the total number of tested
samples with a negative gold standard result.  Inconclusive
results will be classified as non-negative for the computation of
the specificity. 

Likewise, inconclusive results will be classified as non-positive
for the computation of the sensitivity. In other words,
inconclusive results may greatly decrease the diagnostic value of
a test.

--
Philippe Glaziou
Pasteur Institute of Cambodia
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