year and site
Gelman suggests that specifying a weakly informative half-Cauchy distribution for the group level standard deviation can overcome the difficulty in estimation experienced when there are few groups and your using a uniform prior (and I presume using Maximum likelihood too). See Gelman & Hill, 2007. Will Morris Masters of Philosophy candidate Vesk Plant Ecology Lab The School of Botany The University of Melbourne Australia Phone: +61 3 8344 0120 http://www.botany.unimelb.edu.au/vesk/
On 26/09/2009, at 23:29, Andrew J Tyre <atyre2 at unlnotes.unl.edu> wrote:
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 at unl.edu http://snr.unl.edu/tyre http://aminpractice.blogspot.com [[alternative HTML version deleted]]
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