[R-sig-eco] Unbalanced data and random effects

[Chris Howden]

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




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-----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!
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--

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

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