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Re: st: using roctab when lower values of 'classvar' indicate higherrisk


From   Gary Longton <glongton@fhcrc.org>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: using roctab when lower values of 'classvar' indicate higherrisk
Date   Wed, 28 Mar 2007 12:43:47 -0700

Morrison Hodges wrote:

The manual says that when using 'roctab', higher values of 'classvar' must
indicate higher risk. I have data using a cardiac variable called ejection
fraction (EF) as 'classvar', which varies from 0.00 to 1.00, with lower
values indicating higher risk. The 'refvar' is 'status', with 0=alive and
1=dead. The lower the EF the more likely the patient is dead.

roctab status ef, graph

produces a curve below the 45 degree line; I want the curve to be above the
45 degree line. What is a legitimate way to make the curve be above the 45
degree line? Exchange 'refvar' values? Subtract 'classvar' values from 1?
Something else? Thanks.
It is only by *convention* that, in ROC methods, larger values of the test result are more indicative of disease (or, in this case, death). You can legitimately satisfy this convention in this situation by redefining your test result as Y = -EF.

Alternatively exchanging the <refvar> values will give you a different and likely unwanted result: a plot of specificity vs. 1-sensitivity in the situation where it is believed that lower test values are more indicative of disease or other outcome?

- Gary

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