Re: st: Predicted survivor curve for model with interactions

[Enzo Coviello <enzo.coviello@tin.it>]

At 08.52 29/07/03 +0100, you wrote:
> A colleague forwarded me the following problem about how to treat
> interaction terms when drawing survivor curves. Any ideas please?
> --------------------------------------
> I have the model
>
> B1 drug + B2 Age + B3  drug*Age
>
> Imagine I want to plot the predicted survivor curves for the following cases
>
> 1) age=35, drug=0
> 2) age 50 drug=0
> 3) age=35  drug=1
> 4) age=50 drug=1
>
> If I go about what you do, I will estimate the model using either the 
> variable
> age-35 or age-50 and then plot the baseline in each case and obtain cases 1)
> and 2). In those cases since I have centered the age variable to 35 or 50,
> that variable assumes value 0 both for age and for age*drug.
> However, if I am interested in comparing those baselines with the case where
> drug=1, it seems to me that only the term B1 (drug) will vary here whereas 
> the
> value of age*drug will continue to be zero since I have defined age to be
> age-35. To sum up, how do I plot cases 3) and 4)?


Maybe it's a naive advice. Anyway:

stcox age35 drug age35drug,bases(S)
stcurve,surv at1(drug=0, age35=0) at2(drug=1, age35=0) at3(drug=1, 
age35=10, age35drug=10) at4(drug=1, age35=-10, age35drug=-10)

Last two lines should represent survival curve at age=45 and age=25 
respectively where drug term is 1, i.e. cases 3) and 4).


Enzo



^^^^^^^^^^^^^^^^
Enzo Coviello
Dipartimento di Prevenzione ASL BA/1
via L. Barbera 27 - 70055 MINERVINO MURGE (BA)
Italy
tel - fax +39 0883 691053
tel (home) +39 0883 695055  
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