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Re: st: Cox PH question
On 3 Feabh 2006, at 20:09, Ricardo Ovaldia wrote:
A reviewer of a manuscript that we recently submitted
to a top medical journal stated that the "independent
variable, known to be positively skewed , be
log-transformed prior to inclusion in a Cox
proportional hazards model". As far as I know there is
no normality assumption for the Cox model,
additionally transforming this complicates the
interpretation of the reported hazard ratios. Am I
missing something or is the reviewer wrong? Other than
for interpretation, is there another reason to
transform an independent variable in the Cox PH model?
The distribution of the variable will affect the ease of
interpretation of the hazard ratios. Given the distribution, a hazard
ratio associated with an n-fold increase in the predictor variable
might make more sense than a 1-unit increase.
My own hunch is that a variable with a funny distribution might
better be re-expressed in, say, deciles or quintiles. This makes the
hazard ratio easily explainable. You can also do this for variables
on different scales, making your hazard ratios comparable across
model terms. And quantiling will deal with zero values, otherwise a
tough problem on a log scale.
Ron�n Conroy
[email protected]
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