Firstly, sorry to reply to all, but I could not find Liz's e-mail in order to respond directly to her.
Hey Liz, I'm not sure what program you're using to run your ordination analyses, nor what ordination you plan to use, but you may like to consult "Analysis of Ecological Communities" by Bruce McCune and James B. Grace. I am not 100% sure if it addresses your particular question but it is a great resource when conducting multivariate ordination analyses and provides a plethora of resources within it that you may find useful. Best of luck to you!!! Melissa H. Friedman, M.S. School of Forest Resources and Conservation University of Florida - IFAS [email protected] ________________________________________ From: Ecological Society of America: grants, jobs, news [[email protected]] on behalf of Nicolas PERU [[email protected]] Sent: Thursday, September 29, 2011 4:41 AM To: [email protected] Subject: Re: [ECOLOG-L] Ordinal vegetation data and mixed effects models Hi Liz, Modelisation of ordinal data is quite difficult. You should read Agresti's book on categorical data to get some important information. If you're a R (or S-plus) user you can also download Laura Thompson's document to accompany Agresti's book. To be simple, a first step consist of using polynomial contrasts to take into account the ordinal component. You can then analyze your data as classical response in any model of your choice but the difference will be that you will look at linear, quadratic or cubic (depending on the number of levels) tendencies and not at differences between the levels of your factor as with a classical nominal variable. Apart from this quite technical point, I have analyze some BB data with a colleague for a river restoration program. I'm quite doubtful about the pertinence of such data as this correspond to important transformation of raw data. Another way of analyzing this data could be to use beta models as the BB scale is bounded. Regards, Nicolas Le Thu, 29 Sep 2011 03:44:43 +0200, Liz Pryde <[email protected]> a écrit: > Hello Ecolog-ers, > > I'm analysing my Braun-Blanquet cover data for a group of vegetation > structure variables (in diff habitat types) and am having trouble > finding a > definitive way to do this legitimately with mixed effects models, ANOVAs > and > ordinations. > > The issue is that the BB scale is an ordinal scale. Many studies have > simply > taken midpoint values of the scale classes and turned the ordinal data > into > a metric and analysed from there. To me this seems a bit dodgy, given > that > there are really only 9 values that these variables can take, rather than > the 100 values that is implicit in the metric percentage of cover. > > I have read Podani's many papers on the issues and potential solutions > for > ordinations but I seem to be finding it more difficult to find literature > with mixed effects models, or any kind of regression-based model. > > Has anyone else encountered these problems? Can anyone suggest some > current > literature on the matter? > > Thank you kindly, > Liz > > > -- Nicolas PERU, PhD 33-(0)4 72 43 28 94 06-88-15-23-10 CNRS, UMR 5023 - LEHNA Université Claude Bernard - Lyon 1 43 Bld du 11 novembre 1918 Rdc Bât Forel 69622 VILLEURBANNE cedex FRANCE
