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

Marcel Gerds marcel.gerds at berkeley.edu
Wed Feb 2 18:48:49 CET 2011

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

Tel.:  +1 510-643-2202
Mobil: +49 176 21302825

E-Mail: marcel.gerds at berkeley.edu
web:    www.marcel-gerds.de

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