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st: Fw: subgroups in cox regression

From   catherine duggan <>
Subject   st: Fw: subgroups in cox regression
Date   Sun, 1 May 2011 03:40:02 +0100 (BST)

I have what is probably a basic question, which is currently defeating me.

I have run Cox regression with death as my outcome: my main exposure of interest 

is a a continuous variable (IGF)

I have other covariates in the model - I will just refer to two of them, race (5 

level) and tumor stage (2 level)

Xi: stcox igf i.race i.stage

I did the main analysis, and then did some exploratory subgroup  analysis, with 
the usual caveats of smaller sample sizes and using a  more conservative P value

Xi: stcox igf i.race if stage==1

However a reviewer gave the comment

' what you might want to do is to construct a model for effect  modification - 
that is compare the model with no subgroup to one with  trends fit for each 
subgroup - a 1 dgree of freedom likelihood ratio  test - do see if the trends 
differ statistically - a test for  homogeneity of trends.  This would be more 

Unfortunately I have no idea how to accomplish this - I wasn't sure whether this 

was a test for interaction or not.

I'd appreciate any guidance

Many thanks

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