Hi!

I’m running glmer (lme4) models with biodiversity data and I’m having trouble with understanding/finding information on how the offset() option is implemented.

Explicitly, I’m wondering if the offset is only implemented on the dependent variable (as I think it is), or does it also affect independent variables in the model (was told this by a stat guy at our department)?

My data is inventories of birds (species richness and abundance) at the scale of whole farms. Thus, each observation has a different inventory area which I am accounting for in the model as: offset = log(INVAREA).

However, as a fixed effect in the model I’ve got the number of different crop types in the inventoried area. As this variable is also affected by inventoried area, I would like to account for this in some way, but I find it difficult to know the best way to do so.

Right now, I have made a linear quadratic function (using lm) of crop number ~ inventoried area + inventoried area^2 to describe how crop number increases with increasing sample size (area). Then, I have subtracted fitted values from observed number of crops and used this measure in the models. Is this a reasonable work around?

Thanks,
Jonas

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Jonas Josefsson (PhD student)
Swedish University of Agricultural Sciences (SLU)
Department of Ecology
Box 7044
750 07 Uppsala
Sweden

Jonas.Josefsson@slu.se<mailto:Jonas.Josefsson@slu.se>
0046 (0)18 672420
0046 (0)703 752366

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