[R] A priori contrast for binomial GLM

[Michael Dewey]

Dear Rula

That is really a statistical question not one for this list but the 
answer is that the fact that they are all zero for that category 
explains it. Search on-line for separation for more details.

Michael

On 18/01/2019 09:56, Rula Domínguez wrote:
> Hello to everyone,
> 
> after much reading I decided to write because I cannot find a solution to
> my question.
> 
> I already did a priori contrasts before for a continuous variable with
> normal distribution. Now I have another variable (burrow), which is
> binomial, and I can do the GLM for it. But when I do the a priori
> contrasts, it has no result in the cases where all data are 0 (is not that
> there are no data, they are just all 0 in a category (treat 30-30), and I
> want to compare this with others that have ones).
>   Data sructure is like this:
> 
>> head(burrow)
>    date day treat psu  sp  burrow
> 1    3   0 30-30  36    B      0
> 2    3   0 30-30  36    B      0
> 3    3   0 15-30  36    B      1
> 4    3   0 15-30  36    B      1
> 5    3   0 15-30  36    B      1
> 6    3   0 10-25  36    B      1
> 
> My model is this:
>> model4B2<-glm(burrow~ treat, family=binomial(link="logit"), data=D4B)
> 
> And I did the contrast like this:
> 
>> require(multcomp)
> #Test contrastes 30 vs all (there are 4 categories to compare)
> k3010R1<-matrix(c(3,-1,-1,-1),1)
> k3010R1
> t3010<-glht(model4B3.2,linfct=k3010R1)
> summary(t3010)
> 
> But is not working and I am sure it should work.
> 
> Could it be because my explanatory variable is cathegorical?
> Or is just not possible to do contrasts for binomial when you have all 0 in
> some cathegory?
> 
> Thank you in advance,
> 

-- 
Michael
http://www.dewey.myzen.co.uk/home.html