r-sig-mixed-models-bounces@r-project.org wrote on 09/26/2009 09:58:09 AM:

> Douglas Bates <bates@stat.wisc.edu> 
> Sent by: r-sig-mixed-models-bounces@r-project.org
> 
> 09/26/2009 10:00 AM
> 
> To
> 
> hadley wickham <h.wickham@gmail.com>
> 
> cc
> 
> r-sig-mixed-models@r-project.org, Andrew J Tyre 
<atyre2@unlnotes.unl.edu>
> 
> Subject
> 
> Re: [R-sig-ME] year and site
> 
> On Sat, Sep 26, 2009 at 9:17 AM, hadley wickham <h.wickham@gmail.com> 
wrote:
> >> in ecology year a study is done in, and the site it is done at, are
> >> obvious random effects.
> >
> > You're picking the years at random? - that's a pretty impressive 
> study design!
> 
> As I'm sure you realize, Hadley, it is the effects that are random,
> not the years.  If I have studied a particular location for the last
> three years and I use a fixed-effect for the years in my model then I
> can "predict" the response for any one of those three years that I
> wish to.  But I have no information on which to base a prediction for
> next year, which is usually what I want to do.  (I am assuming the
> year is being modeled as a categorical variable rather than in terms
> of trends.)
> 
> Using random effects for the year allows me to characterize the
> variability between years and that does help me in characterizing the
> variability for the next year.

Right - there might be a trend, but often as not the amount of variation 
between years swamps it completely if you only have 2 or 3 years, so I 
typically am using year as a categorical variable. 

I really like the explanation using the need to predict at some future 
year or siteyear - I hadn't thought to express it in those terms. 


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