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Re: st: diagt and accuracy


From   Ronán Conroy <rconroy@rcsi.ie>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: diagt and accuracy
Date   Thu, 09 Jun 2005 16:42:31 +0100

Heather Gold wrote:

Oops, the first line was cut off.
I meant to write that diagt defines accuracy as (sensitivity+specificity)/2.

The help file says

The ROC (Receiver Operating Characteristic curve) area
is (for a simple test) the average of sensitivity and specificity.

- not the same thing. -diagt- doesn't provide accuracy in its output.

>Has anyone heard of accuracy defined as
>(prevalence*sensitivity+(1-prevalence)*specificity) ? This is like a
>weighted average that incorporates prevalence and might be helpful with a
>dichotomous diagnostic test (ie, rather than ROC). If so, is there a
>standard depending on one's field, eg, medicine?
>

There are two sorts of questions you can ask about a test: ones that relate to its ability to identify people and ones that relate to the probability that a result is correct.

Sensitivity/specificity are the proportion of people with/without the condition who are correctly identified. Positive/negative predictive values are the proportion of positive/negative tests which are correct. These latter depend on prevalence, while sensitivity and specificity do not.

It follows that there are two ways of conceptualising overall test performance - proportion of people correctly identified and proportion of tests that are correct. In each case, however, this comes out as the sum of true positives and true negatives over the total N and so is independent of prevalence.

What did you have in mind that the adjustment would accomplish?

--

Ronan M Conroy (rconroy@rcsi.ie) Senior Lecturer in Biostatistics Royal College of Surgeons Dublin 2, Ireland +353 1 402 2431 (fax 2764) -------------------- Just say no to drug reps http://www.nofreelunch.org/

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