Unbalanced data and random effects
Hi Krysztof, Did you have a specific section of Zuur et als book in mind? I've pulled it off my shelf and tried looking up shrinkage, unbalanced design, design, etc in the index but couldn't find anything relevant. I'm sure it's in there, but it?s a rather large book to read in 1 go!! Chris Howden B.Sc. (Hons) GStat. Founding Partner Evidence Based Strategic Development, IP Commercialisation and Innovation, Data Analysis, Modelling and Training (mobile) 0410 689 945 (skype) chris.howden chris at trickysolutions.com.au Disclaimer: The information in this email and any attachments to it are confidential and may contain legally privileged information.?If you are not the named or intended recipient, please delete this communication and contact us immediately.?Please note you are not authorised to copy, use or disclose this communication or any attachments without our consent. Although this email has been checked by anti-virus software, there is a risk that email messages may be corrupted or infected by viruses or other interferences. No responsibility is accepted for such interference. Unless expressly stated, the views of the writer are not those of the company. Tricky Solutions always does our best to provide accurate forecasts and analyses based on the data supplied, however it is possible that some important predictors were not included in the data sent to us. Information provided by us should not be solely relied upon when making decisions and clients should use their own judgement. -----Original Message----- From: r-sig-ecology-bounces at r-project.org [mailto:r-sig-ecology-bounces at r-project.org] On Behalf Of Krzysztof Sakrejda Sent: Thursday, 17 October 2013 12:29 AM Cc: r-sig-ecology at r-project.org Subject: Re: [R-sig-eco] 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/038 7874577/ref=la_B001JRWU88_1_2/192-3027843-3405263?s=books&ie=UTF8&qid=1381 929893&sr=1-2 or here: http://www.amazon.com/Analysis-Regression-Multilevel-Hierarchical-Models/d p/052168689X/ref=sr_1_3?s=books&ie=UTF8&qid=1381929942&sr=1-3&keywords=gel man+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 _______________________________________________ R-sig-ecology mailing list R-sig-ecology at r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology