Binaries as Covariates?

[Roy Money]

I have a question about model specification for a Mixed Model analysis.

It is a repeated measures design and I have two binary variables, assignment (placebo vs active med) and age (under 48 and over 48).
I have a model with several interaction terms and want to predict the marginal means for time by assign by age which I have done.
I do not use the EMM feature because this does not make allowance for the covariate terms in my model.

So the question arises -
Is there any reason I should specify the binary variables of assignments and age only as factors, or is it meaningful to  enter them as covariates?
The difference in specification doesn't affect the predicted values for means but it does affect the table of fixed effects.
Since different F values and df are obtained by these two methods I am presented with very different interpretations of the effects.

Any guidance would be appreciated.

Thanks .
Roy



 
 

Roy Money M.S.
Biostatistician
Treatment Research Program
Connecticut Mental Health Center
Yale University
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