[R] Forcing model parameters to a constant (in GLMs)

[David Reitter]

Hi,

I'm looking for a way to bind specific model parameters to a constant  
(in my case: 0) before fitting the model. I'm fitting multinomial  
logistic regression models (via glm()).
Assuming a model with a categorial response variable c  {c1,c2,c3}  
that is a matrix like

c2:    b2  b2.1  b2.2  b2.3
c3:    b3  b3.1  b3.2  b3.3,

I would like to force,  e.g. b2.1, b3.1 and b2.3 to 0 before  
estimating the other parameters.

I'm aware of a similar - unresolved - question on list list back in  
July:

https://stat.ethz.ch/pipermail/r-help/2005-July/075042.html

The "offset" in an interaction term doesn't seem to do what I want -  
as far as I can see, it manipulates the intercept, and it's additive  
(i.e. an intercept is estimated anyways).

Thanks
David

--
David Reitter - ICCS/HCRC, Informatics, University of Edinburgh