[R-sig-ME] glmer.nb offset problem
Dagmar
Ramgad82 at gmx.net
Tue Jul 25 17:52:11 CEST 2017
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
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