[R] use "factor" for categorical covariate in Cox PH model

From: Linda Lei <llei_at_bccrc.ca>
Date: Tue 18 Jul 2006 - 09:57:47 EST


Hi All,  

I'm learning the R codes for Cox PH modeling. Could I ask you what the function of "factor" in modeling? Thank you!  

When dealing with the categorical covariates (for example 3 groups), it will come out different results if we add the command "factor" in front of the categorical covariate or not: if we don't add "factor", there is only one hazard ratio; if we add "factor", there are two hazard ratios.  

So does the "factor" actually create some dummy variables for the calculation?    

Here is the code example:  

curve2=survfit(Surv(CR, statcr) ~ factor(X1),data=dat1)

where X1 have three categories.  

Thanks!  

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