Mixed Model Specification

Dear Tom,

Here are my thoughts on your question:
In mixed effect models the aim is to account for the variation present 
in the random terms when the standard error around the coefficients of 
the fixed effect part is computed. As you have only one replicate per 
site you cannot assess this variation. For this dataset I would drop the 
site variable since any statistical method I know of need some kind of 
assessment of the variation due to a variable to derive inference.

Hope this help,
Lionel

Dear All

I have a question about the use of a mixed effects model. I have presence/absence data for a mussel species collected at 25 sites. I wish to relate the presence/absence to a number of environmental variables and also want to take into account site. Is it feasible to use site as a random effect as I have only one replicate per site   e.g.

M1<-glmer(Presence ~ Substrate, (1 | Site), family = binomial, data = data)

Best wishes

Tom

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