Thanks for your reply, but I still couldn't solve the problem. I am using the package statmod, and I need the AIC because I want to use the step function (I am modelling many species, so I would prefer to do the step automatically). I can't find the tweedie package in the R packages list and I don't know how to download it from the package source available in *http://cran.r-project.org/src/contrib/Descriptions/tweedie.html * <http://cran.r-project.org/src/contrib/Descriptions/tweedie.html>. However, after running the tweedie.R file from that package source I am able to use the dtweedie() function, but still I didn't figure out a way to do the step (or to get the AIC from the glm command).
Thank you again for your help; Catarina On 13/08/07, Gordon Smyth <[EMAIL PROTECTED]> wrote: > > Dear Catarina, > > I prefer to leave the AIC value as NA for the tweedie GLM family > because it takes extra time to compute and is only occasionally > wanted. It's easy to compute the AIC yourself using the dtweedie() > function of the tweedie package. > > Best wishes > Gordon > > At 03:05 AM 14/08/2007, Catarina Miranda wrote: > >Dear Gordon; > > > >I have also sent this email to R help mailing list, so I apologize > >for duplicated mailing. > >I am modelling densities of some species of birds, and I have a > >problem with a great amount of zeros. > >I have decided to try GLMs with the tweedie family, but in all the > >models I have tried I got an NA for the AIC value. > >Just to check the problem I've compared the a glm using the > >Gaussian family with the identity link and a glm using the tweedie > >family with var.power=0 and link.power=1. These are equal, as > >expected, except the fact that the tweedie output gives me an NA for the > AIC. > >Could you help me with this problem? > >Below you can find the two outputs I refer. > > > >Best Wishes; > > > >Catarina > > > > > summary(glm(formula=ACIN~DIST_REF+DIST_H2O+DIST_OST+ > > COTA+H2O_SUP+vasa,family=gaussian(link="identity"))) > >Call:glm(formula = ACIN ~ DIST_REF + DIST_H2O + DIST_OST + COTA > >+ H2O_SUP + vasa, family = gaussian(link = "identity")) > >Deviance > >Residuals: Min 1Q Median 3Q Max > >-0.112792 -0.042860 -0.021113 -0.006311 1.551824 > >Coefficients: Estimate Std. Error t value > >Pr(>|t|) (Intercept) > >-6.625e-02 5.454e-02 -1.215 0.2256 DIST_REF 3.581e-06 > >1.336e-05 0.268 0.7889 DIST_H2O - > > 3.168e-05 1.527e-05 -2.074 0.0391 > >*DIST_OST -1.799e-05 1.953e-05 -0.921 0.3579 COTA > >5.648e-04 2.470e-04 2.287 0.0230 > >*H2O_SUP -2.172e-04 3.994e-04 -0.544 0.5870 vasa > >3.695e-02 4.573e-02 0.808 0.4199 ---Signif. codes: 0 '***' > >0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 > >(Dispersion parameter for gaussian family taken to be 0.02151985) > > Null deviance: 5.6028 on 257 degrees of freedomResidual > > deviance: 5.4015 on 251 degrees of freedomAIC: -249.33 > >Number of Fisher Scoring iterations: 2 > > > > > > > summary(glm(formula=ACIN~DIST_REF+DIST_H2O+DIST_OST+ > > COTA+H2O_SUP+vasa,control= > > glm.control(maxit=750),family=tweedie(var.power=0, link.power=1))) > >Call:glm(formula = ACIN ~ DIST_REF + DIST_H2O + DIST_OST + COTA > >+ H2O_SUP + vasa, family = tweedie( var.power = 0, link.power = > >1), control = glm.control (maxit = 750)) > >Deviance > >Residuals: Min 1Q Median 3Q Max > >-0.112792 -0.042860 -0.021113 -0.006311 1.551824 > >Coefficients: Estimate Std. Error t value > >Pr(>|t|) (Intercept) - > >6.625e-02 5.454e-02 -1.215 0.2256 DIST_REF 3.581e-06 > >1.336e-05 0.268 0.7889 DIST_H2O -3.168e-05 1.527e-05 > >-2.074 0.0391 > >*DIST_OST -1.799e-05 1.953e-05 -0.921 0.3579 COTA > >5.648e-04 2.470e-04 2.287 0.0230 > >*H2O_SUP -2.172e-04 3.994e-04 -0.544 0.5870 vasa > >3.695e-02 4.573e-02 0.808 0.4199 ---Signif. codes: 0 '***' > >0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 > >(Dispersion parameter for Tweedie family taken to be 0.02151985) > > Null deviance: 5.6028 on 257 degrees of freedomResidual > > deviance: 5.4015 on 251 degrees of freedomAIC: NA > >Number of Fisher Scoring iterations: 2 > > > > > > [[alternative HTML version deleted]] ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.