Unbalanced data and random effects
On Wed, Oct 16, 2013 at 6:41 AM, <v_coudrain at voila.fr> wrote:
Dear all, I performed a census of insects at different sites and measured there size. I would like to know if size is related to an environmental factor. I modelled the size as a fonction of the factor with site as a random variable to account for within-site variability. However I have strong unbalanced data with some sites having only two individuals and others up to 100. Is having site as a random factor sufficient to deal with this strong data unbalance?
I'm not sure what you mean by "deal with", but reading about shrinkage in random effects models in any decent source would probably be a fine start for you, either here: http://www.amazon.com/Effects-Extensions-Ecology-Statistics-Biology/dp/0387874577/ref=la_B001JRWU88_1_2/192-3027843-3405263?s=books&ie=UTF8&qid=1381929893&sr=1-2 or here: http://www.amazon.com/Analysis-Regression-Multilevel-Hierarchical-Models/dp/052168689X/ref=sr_1_3?s=books&ie=UTF8&qid=1381929942&sr=1-3&keywords=gelman+bayesian The short answer is that the site effect will shrink toward the average site effect for sites with few individuals. Krzysztof
The residual fit of the data is quite bad, certainly because of the strong difference in variance among sites.
Would anybody have some advice? Thank you!
___________________________________________________________ Les pr?visions m?t?o pour aujourd'hui, demain et jusqu'? 8 jours ! Voila.fr http://meteo.voila.fr/ _______________________________________________ R-sig-ecology mailing list R-sig-ecology at r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
Krzysztof Sakrejda Organismic and Evolutionary Biology University of Massachusetts, Amherst 319 Morrill Science Center South 611 N. Pleasant Street Amherst, MA 01003 work #: 413-325-6555 email: sakrejda at cns.umass.edu