Hi
I am doing a survival analysis and have run into a couple of things that I was hoping I could get some advice on.
 
The first thing is that when I run an ordinary Cox regression in R I get the same results that I do using Stata (provided that I specify the Efron method for handling ties) but I also get the following warning message:
 
"Warning message:
In coxph(Surv1 ~ Out + site + haem) :
  X matrix deemed to be singular; "
 
Is this warning important and definitely applicable? I would have thought that if there was a problem with the data I would also get some sort of warning in Stata? 
 
The second problem concerns the estimation of a weighted Cox regression i.e I am using coxph with the weights option (I have calculated the weights in Stata). Basically I am getting the following error message:
 
"Error in fitter(X, Y, strats, offset, init, control, weights = weights,  : 
  NA/NaN/Inf in foreign function call (arg 10)
In addition: Warning message:
In fitter(X, Y, strats, offset, init, control, weights = weights,  :
  Ran out of iterations and did not converge"
 
Is there anything I can do to try and get around this? I tried reducing the model from one that initially had 7 parameters to one with just one but kept on getting the same message.
 
BTW I have run the same commands on a different outcome variable where I did not get either of these problems so I am aware that they don't always appear and are related to the data.
 
Thanks very much
 
Greg

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