Thank you very much Mollie. Best,
Manuel 2013/2/3 Mollie Brooks <mbro...@ufl.edu> > Hi Manuel, > This means that your data is closer to Poisson. > Here is an example where I simulate Poisson data and try to fit the NB > distribution. I get behavior from glm.nb that is similar to your results > (large dispersion parameter and warning about iteration limit reached). > > x=rep(1:5, 10) > y=rpois(n=length(x), lambda=exp(x*.5+1)) > dat=data.frame(x=x,y=y) > library("MASS") > m1=glm.nb(y~x, dat) > m2=glm(y~x, dat, family="poisson") > > Try using glm instead of glm.nb > > cheers, > Mollie > > Mollie Brooks > Postdoctoral Researcher, Ponciano Lab > Biology Department, University of Florida > http://people.biology.ufl.edu/mbrooks > > > On 3 Feb 2013, at 10:16 AM, Manuel Spínola wrote: > > Dear list members, > > I am fitting a negative binomial model but I get a very large dispersion > parameter. Why is that? > > quine.nb2 <- glm.nb(Pt ~ Agua + Dist10, data = Abundancia) > > summary(quine.nb2) > > Deviance Residuals: > Min 1Q Median 3Q Max > -2.20911 -0.67157 0.04411 0.35695 1.65524 > > Coefficients: > Estimate Std. Error z value Pr(>|z|) > (Intercept) -0.88354 0.43982 -2.009 0.0445 * > Agua[T.negra] 0.89319 0.43727 2.043 0.0411 * > Dist10 0.39416 0.04387 8.984 <2e-16 *** > --- > Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 > > (Dispersion parameter for Negative Binomial(67552.88) family taken to be 1) > > Null deviance: 128.567 on 15 degrees of freedom > Residual deviance: 17.005 on 13 degrees of freedom > AIC: 67.058 > > Number of Fisher Scoring iterations: 1 > > > Theta: 67553 > Std. Err.: 1428117 > Warning while fitting theta: iteration limit reached > > 2 x log-likelihood: -59.058 > > > Best, > > Manuel > > -- > *Manuel Spínola, Ph.D.* > > Instituto Internacional en Conservación y Manejo de Vida Silvestre > Universidad Nacional > Apartado 1350-3000 > Heredia > COSTA RICA > mspin...@una.ac.cr > mspinol...@gmail.com > Teléfono: (506) 2277-3598 > Fax: (506) 2237-7036 > Personal website: Lobito de río < > https://sites.google.com/site/lobitoderio/> > Institutional website: ICOMVIS <http://www.icomvis.una.ac.cr/> > > [[alternative HTML version deleted]] > > _______________________________________________ > R-sig-ecology mailing list > R-sig-ecology@r-project.org > https://stat.ethz.ch/mailman/listinfo/r-sig-ecology > > > -- *Manuel Spínola, Ph.D.* Instituto Internacional en Conservación y Manejo de Vida Silvestre Universidad Nacional Apartado 1350-3000 Heredia COSTA RICA mspin...@una.ac.cr mspinol...@gmail.com Teléfono: (506) 2277-3598 Fax: (506) 2237-7036 Personal website: Lobito de río <https://sites.google.com/site/lobitoderio/> Institutional website: ICOMVIS <http://www.icomvis.una.ac.cr/> [[alternative HTML version deleted]]
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