[R] Survival analysis with no events in one treatment group

From: John Field <JohnField_at_ozemail.com.au>
Date: Mon, 31 Dec 2007 12:45:42 +1030

I'm trying to fit a Cox proportional hazards model to some hospital admission data. About 25% of the patients have had at least one admission, and of these, 40% have had two admissions within the 12 month period of the study. Each patients has had one of 4 treatments, and one of the treatment groups has had no admissions for the period. I used:

surv.obj<-Surv(time=time1,time2=time2,event=event,type="counting") model<-coxph(surv.obj~Treatment+cluster(Subject))

and, as explained in the coxph help page, I get a warning message about convergence because the MLE of one of the coefficients is infinite since there are no admissions in one group.

I'm looking for suggestions about how to proceed with an analysis of these data. I'd prefer not to ignore the fact that there are multiple admissions, but any alternative ideas I have at the moment do this.

Many thanks,
John Field

Faculty of Health Sciences Statistical Support Service The University of Adelaide, Australia 5005

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