Friday, February 03, 2006

st: RE: Cox PH question

It's not a question of a normality assumption--it's a question of whether your variable of interest relates linearly or logarithmically to survival.

Dan Waxman

-----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Ricardo Ovaldia Sent: Friday, February 03, 2006 3:09 PM To: statalist@hsphsun2.harvard.edu Subject: st: Cox PH question

Dear all,

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?

Thank you in advance, Ricardo.

Ricardo Ovaldia, MS Statistician Oklahoma City, OK

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