Re: [R] Predicted Cox survival curves - factor coding problems..

From: Terry Therneau <therneau_at_mayo.edu>
Date: Mon, 07 May 2007 07:53:29 -0500 (CDT)


  The combination of survfit, coxph, and factors is getting confused. It is not smart enough to match a new data frame that contains a numeric for sitenew to a fit that contained that variable as a factor. (Perhaps it should be smart enough to at least die gracefully -- but it's not).

   The simple solution is to not use factors.    

site1 <- 1*(coxsnps$sitenew==1)
site2 <- 1*(coxsnps$sitenew==2)
test1 <- coxph(Surv(time, censor) ~ snp1 + sex + site1 + site2 + gene +
	  eth.self + strata(edu), data= coxsnps)
	  
	 output

profile1 <- data.frame(snp1=c(0,1), site2=c(0,0), sex=c(0,0),  
	               site1=c(0,0), site2=c(0,0), geno=c(0,0) eth.self=c(0,0))
plot(survfit(test1, newdata=profile1))

 Note that you do not have to explicitly make "edu" a factor. Since it is included in a strata statement, the coxph routine must treat it as discrete groups.

        Terry Therneau



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