[R] A question about stepwise procedures: step function

[Li, Jia]

Dear all,

I tried to use "step"  function to do model selection, but I got an error massage.  What I don't understand is that data as data.frame worked well for my other programs, how come I cannot make it run this time. Could you please tell me how I can fix it?

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>all<-data.frame(z1,z2,z3)

>fit.model.all<- coxph(Surv(t,cen) ~z1+z2+z3,data=all)

> reg.model.all<-step(fit.model.all)
Start:  AIC= 689.1 
 Surv(t, cen) ~ z1 + z2 + z3 
Error in as.data.frame.default(data) : cannot coerce class "function" into a data.frame
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Thanks a lot!
 
Jia