[R] Formula with no intercept

Doran, Harold HDoran at air.org
Wed Apr 16 22:02:02 CEST 2008

R may not be giving you what you want, but it is doing the right thing.
You can change what the base category is through contrasts but you can't
get the marginal effects for every level of all factors because this
creates a linear dependence in the model matrix. 

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Gang Chen
> Sent: Monday, April 14, 2008 5:38 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Formula with no intercept
> I'm trying to analyze a model with two variables, one is 
> Group with two levels (male and female), and other is Time 
> with four levels (T1, T2, T3 and T4). And for the convenience 
> of post-hoc testing I wanted to consider a model with no 
> intercept for factor Time, so I tried formula
> Group*(Time-1)
> However this seems to give me the following terms in the model
> GroupMale, GroupFemale, TimeT2, TimeT3, TimeT4, 
> GroupMale:TimeT2, GroupMale:TimeT3, GroupMale:TimeT4, 
> GroupFemale:TimeT2, GroupFemale:TimeT3, GroupFemale:TimeT4
> which is not exactly what I wanted. Also it seems (Group-1)*Time and
> (Group-1)*(Time-1) also give me exactly the same set of terms 
> as Group*(Time-1).
> So I have some conceptual trouble understanding this. And how 
> could I create a model with terms including all the levels of 
> factor Time?
> Thanks,
> Gang
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