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st: Transformed values in logistic regression


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Transformed values in logistic regression
Date   Thu, 26 Aug 2004 20:27:05 -0700 (PDT)

Dear all,

Please pardon this non-Stata question, but I have not
found a satisfactory resolution. 

We recently submitted a manuscript for publication and
received a favorable reply, except that one of the
reviewers criticized a transformation that we
performed on a continuous variable prior to fitting a
logistic regression model. 

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”

Is he correct?  Isn’t the coefficient estimated such
that the predicted values are within the range of the
data and this only a problem if you attempt to
extrapolate beyond the data range? What I am missing?

Thank you very much,
Ricardo.   


=====
Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK


	
		
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