I have a general follow-up question to everyone.
Is this "legal" to include interaction terms while excluding the main
effects?

Dimitri

On Wed, Feb 2, 2011 at 4:17 PM, Marcel Gerds <marcel.gerds@berkeley.edu>wrote:

> I withdraw the question. I have just figured it out myself.
> One has only to create a variable which includes the interaction:
>
> interact <- cost_a / income
>
> This new variable can then be included in the model without having the
> respective main effect in the model.
>
> mlogit.model <- mlogit ( choice ~ interact + cost_b , data = data _ ml )
>
> Best regards,
> Marcel
>
>
>  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
> Mobil: +49 176 21302825
>
> E-Mail: marcel.gerds@berkeley.edu
> web:    www.marcel-gerds.de
>
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