Message: 1 Date: Mon, 8 Feb 2010 11:02:37 +0100 From: "ONKELINX, Thierry"<Thierry.ONKELINX at inbo.be> To: "Peter Solymos"<solymos at ualberta.ca>, "Nathan Lemoine" <lemoine.nathan at gmail.com> Cc: r-sig-ecology at r-project.org Subject: Re: [R-sig-eco] multiple regression Message-ID: <2E9C414912813E4EB981326983E0A10406FDECE6 at inboexch.inbo.be> Content-Type: text/plain; charset="iso-8859-1" Peter, I would think that the species richness is binomial distributed. Since there is a maximum number of species that can be present. Therefore I would model it like glm(cbind(number.present, number.absent) ~ covariates, family = binomial)
Interesting thought. Maybe correct. But there are a few things to think about: 1. You have to assume that sampling was such that all species out there have ended up in the data. Formulated differently...you need to know the N_i (maximum number of possible species) in the B(p_i,N_i). 2. Not sure how the variance structure would work out; Poisson or NB versus Binomial. I guess it may work out ok. 3. B(p_i,N_i) means N_i independent (!) trials, each (!) with probability p_i of success. But what happens if these trials (=species) are not independent? Anyway....it is not my problem....but you would need to think about these things. Alain
HTH, Thierry ---------------------------------------------------------------------------- ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek team Biometrie& Kwaliteitszorg Gaverstraat 4 9500 Geraardsbergen Belgium Research Institute for Nature and Forest team Biometrics& Quality Assurance Gaverstraat 4 9500 Geraardsbergen Belgium tel. + 32 54/436 185 Thierry.Onkelinx at inbo.be www.inbo.be To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of. ~ Sir Ronald Aylmer Fisher The plural of anecdote is not data. ~ Roger Brinner The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data. ~ John Tukey
Dr. Alain F. Zuur First author of: 1. Analysing Ecological Data (2007). Zuur, AF, Ieno, EN and Smith, GM. Springer. 680 p. URL: www.springer.com/0-387-45967-7 2. Mixed effects models and extensions in ecology with R. (2009). Zuur, AF, Ieno, EN, Walker, N, Saveliev, AA, and Smith, GM. Springer. http://www.springer.com/life+sci/ecology/book/978-0-387-87457-9 3. A Beginner's Guide to R (2009). Zuur, AF, Ieno, EN, Meesters, EHWG. Springer http://www.springer.com/statistics/computational/book/978-0-387-93836-3 Other books: http://www.highstat.com/books.htm Statistical consultancy, courses, data analysis and software Highland Statistics Ltd. 6 Laverock road UK - AB41 6FN Newburgh Tel: 0044 1358 788177 Email: highstat at highstat.com URL: www.highstat.com URL: www.brodgar.com