[R] Linear mixed effects model ?
dieter.menne at menne-biomed.de
Tue Oct 6 17:26:20 CEST 2009
Daniel Perkins wrote:
> Ideally we would do an ANCOVA to test for differences in slope or
> intercepts for the different streams. However as there were repeated
> measures and unequal n and unbalanced design, I have used a linear mixed
> effect model (from nlme package in R) in the form:
> model <- lme (Rate ~ Temp* Stream, random = ~ Time|Replicate)
This should rather be
model <- lme (Rate ~ Temp* Stream, random = ~ 1|Time/Replicate)
Always start your model with the more parsimonious "equal slopes" assumption
denoted by 1|. When you place the Time variable as you did, it estimates
separate slopes for Time, which could be ill conditioned.
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