On Wed, 16 Apr 2008, Doran, Harold wrote: > 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.
I suspect that Time*Group - 1 or Time*(Group-1) come closer to the aim. It is the first factor in the model which is coded without contrasts in a no-intercept model. Once you include interactions I think the 'convenience' is largely lost, and packages such as multcomp can post-hoc test any (coherent) set of hypotheses you choose, irrespective of the model parametrization. >> -----Original Message----- >> From: [EMAIL PROTECTED] >> [mailto:[EMAIL PROTECTED] On Behalf Of Gang Chen >> Sent: Monday, April 14, 2008 5:38 PM >> To: [EMAIL PROTECTED] >> 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 >> >> ______________________________________________ >> R-help@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide >> http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. >> > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
