[R] Problems with coxph and survfit in a stratified model with interactions

[rm]

Hi Chris,

Yes, that is exactly the model I want to fit, i.e. a separate 'baseline'
hazard for each stratum (defined by cov1) and a coefficient for cov2 that is
different for each stratum. 

Is suspect that there is a problem with the following line of your code. 

sCox <- coxph(Surv(time, status) ~ strata(cov1) + cov2 + cov1 :cov2,
data=lung) # warning, X matrix singular 

The reason why I suspect is that in an earlier  post
<http://r.789695.n4.nabble.com/Re-Problems-with-coxph-and-survfit-in-a-stratified-model-with-interactions-td4646171.html>  
Terry wrote that ” ..by using ~ strata(cov1) + cov1:cov2 you fooled the
program into thinking that there was no strata by covariate interaction, ” 

Best regards,
Roland




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