
I am using mixed effect models (library nlme, lme) to evaluate  factors that influence bycatch of hake in a shrimp fishery. Data are collected though an on-board observer program and  I decided to treat the vessel id and the observer id as random effects. A priori I would expect the observer effect to be independent of  vessel.  However, I did not find a way to incorporate both factors as independent, unnested random effects; I could only estimate the effects as “vessel” and “observer in vessel” (nested). 
 
Is it possible  to incorporate two random effects unested?
 
Lic. María Eva Góngora
 
 
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