Hi all,

in ecology year a study is done in, and the site it is done at, are 
obvious random effects. However, for most student projects, and indeed, 
even larger projects, having more than 2-3 years of data is unusual. 
Having more than 2-3 sites is more common, but even then the number is 
very limited. This means that estimating a random effect of year crossed 
with site is difficult. The solution that I've tried a couple times is to 
create a "siteyear" random effect, where each year at each site is a 
different level. This gives many more levels with which to estimate 
variances, and seems to work well. The only downside that I've been able 
to come up with is that if there are "site" effects that are consistent 
through time, or year effects consistent across space, then this approach 
misses separating them, at a minimum. What I'm concerned about is that it 
might lead to bias as well - I don't think it will, but I'd rather be 
certain! 

I have used the "fallback" of fitting year and/or site as a fixed effect, 
but I'm hearing some concern from reviewers who've been sold on the random 
effects idea convincingly enough to reject a paper that doesn't treat year 
or site as random without recognizing that it doesn't help when the number 
of levels is low. Also, if there is a continuous covariate whose effect 
varies by siteyear, the fixed effect interaction term gets ugly, fast. 

Any discussion or insights appreciated! 

thanks,



Drew Tyre

School of Natural Resources
University of Nebraska-Lincoln
416 Hardin Hall, East Campus
3310 Holdrege Street
Lincoln, NE 68583-0974

phone: +1 402 472 4054 
fax: +1 402 472 2946
email: atyre2@unl.edu
http://snr.unl.edu/tyre
http://aminpractice.blogspot.com
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