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

[John Field]

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