Wednesday, November 08, 2006

random effects

Hi Fransesc,
you seem to have only a small number of levels for the plot effects.
The consequence will be that estimates of variance between these random effects are very imprecise, and that it is not very solid to state that these levels are a representative sample from a large population of plot levels.
I would fit all effects as fixed.
Continuous variables such as time cannot define levels random effects, but they can appear in random regressions. Anyway, if you want to fit higher-order models of time, all you have to do is make new variables, like
timesq<-time^2
and add them to the model.
In a model with interactions of these time variables with treatment (or other factors), different polynomials of time will be fit for each treatment group.
Hope that helps,
Tom

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