R help - Mar 2004 - Constraining coefficients

Hello All:

I have a binomial model with one covariate, x1, treated as a factor with 
3 levels. The other covariate is measured x2 <- 1:30. The response, y, 
is the proportion of successes out of 20 trials.

glm(cbind(y, 20 - y) ~ x1 * x2, family = binomial)

Now, I would like to constrain the cofficients on 2 levels of the 
factor, x1, to be identical and test the difference between these models 
by a likelihood ratio test.

How can I get glm() to constrain the coefficients on 2 levels to be the 
same?

Thanks,
ANDREW

How about collapsing those two levels into one?  Wouldn't that work?

Cheers,
Andy
> From: Andrew Criswell
> 
> Hello All:
> 
> I have a binomial model with one covariate, x1, treated as a 
> factor with 
> 3 levels. The other covariate is measured x2 <- 1:30. The 
> response, y, 
> is the proportion of successes out of 20 trials.
> 
> glm(cbind(y, 20 - y) ~ x1 * x2, family = binomial)
> 
> Now, I would like to constrain the cofficients on 2 levels of the 
> factor, x1, to be identical and test the difference between 
> these models 
> by a likelihood ratio test.
> 
> How can I get glm() to constrain the coefficients on 2 levels 
> to be the 
> same?
> 
> Thanks,
> ANDREW

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