[R-sig-ME] Modelling random effects with SITE, YEAR and SPECIES
drbn at yahoo.com
Wed Apr 29 22:30:46 CEST 2009
First, sorry for the english and the basic questions. I'm using mixed models (lme4 package) to analyse variability in 13 SPECIES of birds observed during 15 YEARS across 5 SITES. All the SPECIES were observed in all the sites in most years.
My fixed effects are A, B, C and Year. I'm interested in the stochastic effect of A, B and C on the dependent variable, but also in a possible linear trend of the dependent variable over time.
My random effects are SPECIES, YEAR and SITE, to control for the effects of nonindependence.
I have a model with SITE, YEAR and SPECIES as crossed random effects like A + B + C + Year + (1|SITE) + (1|YEAR) + (1|SPECIES).
My questions are:
1) Is this model correct? It is correct to model YEAR both as random effect and fixed effect? Is there the possibility that the variance accounted for by the random effect could robbing year as a fixed effect of explanatory power?
2) It is meaningful, instead, to model YEAR as repeated measure, if the experimental unit were species within sites?
Thanks in advance
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