[R-sig-ME] glmer.nb offset problem

[Dagmar]

Dear all,

I am having a problem with a negative binomial GLMM using an offset.

I have GPS fixes from animals and calculated their homerange. Now I want 
to test if there are differences in the size of the homeranges between 
seasons of the year.

I did a simple linear regression analysis between size of home range and 
sample size (number of GPS fixes) and unfortunately found a slight 
positive relationship: The bigger my samle size the larger is the 
homerange. That is why I thought to use sample size as an offset in my 
negative binomial glmm.

My code is:

glmer.nb (Homerangearea ~SeasonOfYear 
+(1|AnimalID)+offset(log(NumberOfGPSfixes)), data=mydataframe)

Without an offset I get a wonderful result with totaly nice fitted 
values. My problem is: When I include the offset my fitted values go 
towards 0. Why is that?

The number of GPS fixes is very different between animals. In one case I 
do only have 60 fixes, in another case I do have 12800. Maybe this 
difference is too big? I do not totally understand what the offset is 
doing to the data. As far as I understood it adds some weighting to some 
data. Is that correct?

In other examples which I found online an offset is only used for small 
numbers. Or is an offset even not the right tool for my problem?

Help would be very much (!!) appreciated.

Tagmarie