[R-sig-DCM] Inclusion of interaction without its main effect

[Marcel Gerds]

Dear fellow DC modellers,

I want to include an interaction term as an explanatory variable without 
having its main effect in the model. For example, I run

mlogit.model <- mlogit ( choice ~ cost_a / income + cost_b , data = data 
_ ml )

where two kind of cost are used as explanatory variables for choosing an 
alternative. Income is a subject-specific variable. However, if I run 
this, R not only includes cost_b and cost_a:income in the model, but 
also cost_a.
Is there a way to suppress the inclusion of a main effect when one wants 
to add only its interaction?
Thanks for any help.

Best regards,
Marcel

-- 
Marcel Gerds, M.Sc.
University of California
Department of Agricultural and Resource Economics
233 Giannini Hall
Berkeley, CA 94720

Tel.:  +1 510-643-2202
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E-Mail: marcel.gerds at berkeley.edu
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