A colleague forwarded me the following problem about how to treat
interaction terms when drawing survivor curves. Any ideas please?
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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)?
Interestingly I cannot find in the literature of survival analysis many
references to interactive variables, any suggestions?
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Stephen
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Professor Stephen P. Jenkins <[email protected]>
Institute for Social and Economic Research (ISER)
University of Essex, Colchester, CO4 3SQ, UK
Tel: +44 (0)1206 873374. Fax: +44 (0)1206 873151.
http://www.iser.essex.ac.uk
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