R help - Nov 2009 - stepAIC(coxph) forward selection

Dear R-Help,

I am trying to perform forward selection on the following coxph model:
>my.bpfs <- Surv(bcox$pfsdays, bcox$pfscensor)
> b.cox <- coxph(my.bpfs ~ Cbase + Abase + Cbave + CbSD + KPS + gender +
as.factor(eor) + Age)>stepAIC(b.cox, scope=list(upper =~ Cbase + Abase +
Cbave + CbSD + KPS + gender + as.factor(eor) + Age, lower=~1),
directionc("forward"))

However the following code returns the full model. How do I write the code
so that forward selection is performed?

Thanks,
Rupa

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On Nov 4, 2009, at 9:26 PM, Rupa Parvataneni wrote:
> Dear R-Help,
>
> I am trying to perform forward selection on the following coxph model:
>
>> my.bpfs <- Surv(bcox$pfsdays, bcox$pfscensor)
>> b.cox <- coxph(my.bpfs ~ Cbase + Abase + Cbave + CbSD + KPS +  
>> gender +
> as.factor(eor) + Age)
I am a bit puzzled by the choice of a maximal model when you are then  
specifying only "forward". Wouldn't that choice imply that you
should
be starting with;

b.cox <- coxph(my.bpfs ~ 1)
> >stepAIC(b.cox, scope=list(upper =~ Cbase + Abase +
> Cbave + CbSD + KPS + gender + as.factor(eor) + Age, lower=~1),  
> direction> c("forward"))
> However the following code returns the full model. How do I write  
> the code
> so that forward selection is performed?
Nothing in this reply should be taken to mean that this is a valid  
approach to model selection. Stepwise methods often are an abdication  
of an analyst's responsibility to use prior scientific knowledge.


-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT