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Re: st: Predicted survivor curve for model with interactions
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
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
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).
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