Hi,
I'm having a bit of trouble with using StepAIC with a coxph model.
Can anybody tell me if there is anything wrong with what I am doing here (I've removed a few of the variables for the purpose of this email, I had about 20 before):
start<- coxph(Surv(entryage2,age_at_death_yr,death)~finih5+prev+magegp+zpluralgp ,data=project_model1)
fmAIC1 <- stepAIC(start,direction="backward",scope=list(lower = ~ 1,upper = ~ finih5+prev+magegp+zpluralgp,data=project_model1))
This is the error I'm getting after it goes through a few times:
Error in stepAIC(start, direction = "backward", scope = list(lower = ~1, :
number of rows in use has changed: remove missing values?
In addition: There were 24 warnings (use warnings() to see them)
The warning messages are all of the form:
1: X matrix deemed to be singular; variable 11 27 28 29 in: coxph(formula = Surv(entryage2, age_at_death_yr, death) ~ prev + ...
These arise because there are no deaths for a particular variable and haven't been a problem when I have been creating my own coxph model.
Thanks, Neil
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