Ben and Ruben, thank you very much for the suggestions. I started the “homework” and I had the impression that I can forget to find a GLMM implementation with Tweedie distribution (in a preliminar GLM, this distribution produced a very decent fit...). However the two-part analysis looks a good option. About the transformation of continous data to discret (to fit a poisson distribution), wouldn't include some subjectivity in the analysis (once the "size of container" of the Ruben example need to be choosed)?
Regards Fred 2011/5/19 Rubén Roa <r...@azti.es>: > This is a well known modeling issue and several approaches are available. > > We have tried the Delta approach with good results. This is essentially a > binomial glm for the presence-absence representation of the data, and > conditional on the Bernoulli variable being 1, a regular continuous > distribution such as Gamma glm for the positive values. Option 3) in Ben > Bolker's list. > > See > > Aitchison, J. 1955. On the distribution of a positive random variable having > a discrete > probability mass at the origin. Journal of American Statistical Association > 50, 901-908 > > Pennington, M. 1983. Efficient estimators of abundance, for fish and plankton > surveys. > Biometrics 39, 281-286 > > Another option is to transform your continuous response into counts. For > example, if you have 0.745 kg of grass from one plot, how many stahdard 100 > ml containers can you fill in a standardized manner with that? That's a > count. Now if you turn your grass biomass into counts and if you are lucky > (not excessive number of zeroes) then maybe a Poisson glm will be good. And > the Poisson does not bug you with nuisance parameters ... > > We tried several things like that, and also the Tweedie distribution (number > 4) in Ben Bolker's list) in this paper: > > Tascheri, R., Saavedra-Nievas, J.C., Roa-Ureta, R. 2010. Statistical models > to standardize > catch rates in the multi-species trawl fishery for Patagonian grenadier > (Macruronus magellanicus) > off Southern Chile. Fisheries Research 105: 200–214 > > Adding a constant to the zeroes is just not right (see p. 324 of the below > quoted article for an authoritative sentence on this matter): > > Venables, W.N., Dichmont, C.M. 20004. GLMs, GAMs and GLMMs: an overview of > theory for > applications in fisheries research. Fisheries Research 70: 319–337. > > HTH > > Rubén > > ____________________________________________________________________________________ > > Dr. Rubén Roa-Ureta > AZTI - Tecnalia / Marine Research Unit > Txatxarramendi Ugartea z/g > 48395 Sukarrieta (Bizkaia) > SPAIN > > > _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
