Thank you Liz, I don't know tweedie, I'll have a look at it, but I have indeed some high values. I know about the problems linked to the arcsine transformation. I won't consider it anyway. I'd like to use either the raw values of pollen grain counts or a logistic quasibinomial model. Best, Valérie
> Message du 02/02/13 à 20h47 > De : "Liz Pryde" > A : "v_coudr...@voila.fr" > Copie à : "Cade Brian" , "r-sig-ecology@r-project.org" > Objet : Re: [R-sig-eco] proportion data with many zeros > > Have you plotted the raw data to have a look at the distribution? > You could try another exponential family distribution like tweedie that has a > mass at zero but is otherwise similar to poisson/gamma - so you're directly modeling the zeroes. It won't work if you have a lot of high values though. > Proportions are tricky. Have a read of the Warton paper (2012/11?) "the > arcsine is asinine". > > Liz > > > > On 02/02/2013, at 6:34 PM, v_coudr...@voila.fr wrote: > > > Thank you very much for this suggestion. In fact I reconsidered my question > > and I am not sure that zero-inflated model is what I need. If I understood > > it properly, > > a zero-inflated model is best suited when we don't know if zero values are > > true or false absences (right?). In my case all zero values are assumed to > > be real > > absence and are therefore informative. However, fitting quasipoisson on raw > > counts or quasibinomial on proportion gives me awful distributions of > > residuals and > > meaningless results. > > > > Valérie > > > > > >> Message du 01/02/13 à 17h22 > >> De : "Cade, Brian" > >> A : v_coudr...@voila.fr > >> Copie à : r-sig-ecology@r-project.org > >> Objet : Re: [R-sig-eco] proportion data with many zeros > >> > >> For a fully parametric approach, you might want to use of zero-inflated > >> beta distribution (e.g., as available in gamlss package), which is designed > >> for zero-inflated proportions. Or for a semi-parametric approach, you > >> could estimated a sequence of quantile regression estimates (e.g., in > >> package quantreg), where some interval (hopefully not to large) of the > >> quantiles will be uninformative because they are massed at the zero values. > >> > >> Brian > >> > >> Brian S. Cade, PhD > >> > >> U. S. Geological Survey > >> Fort Collins Science Center > >> 2150 Centre Ave., Bldg. C > >> Fort Collins, CO 80526-8818 > >> > >> email: brian_c...@usgs.gov > >> tel: 970 226-9326 > >> > >> > >> > >> On Fri, Feb 1, 2013 at 1:30 AM, wrote: > >> > >>> Dear all, I am trying to test how the proportion of pollen of different > >>> plants found in the brood cells of a wild bee changes over time. I > >>> conducted 4 sampling sessions > >>> (thus time is a factor with 4 levels) and collected several pollen samples > >>> for each time point (300 pollen grains counted for each sample). I thought > >>> about applying a > >>> quasi-binomial glm: > >>> > >>> y = cbind(total pollen - pollen of plant X, pollen of plant X) > >>> > >>> glm(y~time, family=quasibinomial) > >>> > >>> The problem is that I have a lot of zero value, because the pollen of some > >>> plants only occurred rarely or very clumped in time. I thought about > >>> applying a zero-inflated > >>> model, but I have never used it and I am not sure if it is suitable for > >>> proportion data. Additionally I wondered if I have to consider the fact > >>> that I don't have the same > >>> number of pollen sample for each date, which makes my design unbalanced. > >>> Thank you in advance for advice. > >>> > >>> Best wishes > >>> Valérie > >>> ___________________________________________________________ > >>> CAN 2013 : résultats et matchs en direct à suivre sur Voila.fr > >>> http://sports.voila.fr/football/can/ > >>> > >>> _______________________________________________ > >>> R-sig-ecology mailing list > >>> R-sig-ecology@r-project.org > >>> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology > > > > ___________________________________________________________ > > CAN 2013 : résultats et matchs en direct à suivre sur Voila.fr > > http://sports.voila.fr/football/can/ > > > > _______________________________________________ > > R-sig-ecology mailing list > > R-sig-ecology@r-project.org > > https://stat.ethz.ch/mailman/listinfo/r-sig-ecology > ___________________________________________________________ CAN 2013 : résultats et matchs en direct à suivre sur Voila.fr http://sports.voila.fr/football/can/ _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology