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Re: st: Transformed values in logistic regression


From   Ronán Conroy <rconroy@rcsi.ie>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Transformed values in logistic regression
Date   Fri, 27 Aug 2004 09:14:26 +0100

Ricardo Ovaldia wrote:


Specifically we were interested in modeling
case-control status as a function of several patient
covariates including serum creatinine which in our
data ranges from 0.11 to 1.98.
Because of skewness and to make the odds ratio
independent of the units measurement, we decided to
log-transform the creatinine values before entering
them into our logistic model. However the reviewer
wrote “Using a log-transform for creatine is absurd
because a 1-unit increase in ln(x) is equivalent to
increasing x by a factor of 2.718 which is in the
realm of impossibility”

I would try expressing creatinine in deciles. This gives a more intuitively appealing measure than taking the log. You can also output, using -adjust- predicted values for, say, the first and last deciles, which give a clear idea of how important creatinine is in your model.
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