[R-sig-eco] proportion data with many zeros

2013-02-01 Thread v_coudrain
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

Re: [R-sig-eco] proportion data with many zeros

2013-02-01 Thread Cade, Brian
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 qu

Re: [R-sig-eco] proportion data with many zeros

2013-02-01 Thread v_coudrain
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 val