R help - Nov 2010 - Define a glm object with user-defined coefficients (logistic regression, family="binomial")

Hi there,

I just don't find the solution on the following problem. :(

Suppose I have a dataframe with two predictor variables (x1,x2) and one 
depend binary variable (y). How is it possible to define a glm object 
(family="binomial") with a user defined logistic function like p(y) = 
exp(a + c1*x1 + c2*x2) where c1,c2 are the coefficents which I define. 
So I would like to do no fitting of the coefficients. Still, I would 
like to define a GLM object because I could then easily use other 
functions which need a glm object as argument (e.g. I could use the 
anova, summary functions).

Thank you very much! Greetings
J?rgen

-- 
-----------------------------------
J?rgen Biedermann
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10961 Berlin-Kreuzberg
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Home: +49 30 250 11 713
e-mail: juergen.biedermann at gmail.com

On Nov 13, 2010, at 7:43 AM, J?rgen Biedermann wrote:
> Hi there,
>
> I just don't find the solution on the following problem. :(
>
> Suppose I have a dataframe with two predictor variables (x1,x2) and  
> one depend binary variable (y). How is it possible to define a glm  
> object (family="binomial") with a user defined logistic function
> like p(y) = exp(a + c1*x1 + c2*x2) where c1,c2 are the coefficents  
> which I define. So I would like to do no fitting of the  
> coefficients. Still, I would like to define a GLM object because I  
> could then easily use other functions which need a glm object as  
> argument (e.g. I could use the anova,
The anova results would have not much interpretability in this  
setting. You would be testing for the Intercept being zero under very  
artificial conditions. You have eliminated much statistical meaning by  
forcing the form of the results.
> summary functions).
# Assume dataframe name is dfrm with variables event, no_event, x1,  
x2, and further assume c1 and c2 are also defined:

dfrm$logoff <- with(dfrm, log(c1*x1 + c2*x2))
forcedfit <- glm( c(event,no_event) ~ 1 + offset(logoff), data=dfrm)

(Obviously untested.)
>
> Thank you very much! Greetings
> J?rgen
>
> -- 
> -----------------------------------
> J?rgen Biedermann

David Winsemius, MD
West Hartford, CT