Christoph Scherber wrote: > Dear all, > > For an introductory course on glm?s I would like to create an example to > show the difference between glm and transformation of the response. For > this, I tried to create a dataset where the variance increases with the > mean (as is the case in many ecological datasets): > > poissondata=data.frame( > response=rpois(40,1:40), > explanatory=1:40) > > attach(poissondata) > > However, I have run into a problem because it looks like the lm model > (with sqrt-transformation) fits the data best: > > ## > > model1=lm(response~explanatory,poissondata) > model2=lm(sqrt(response+0.5)~explanatory,poissondata) > model3=lm(log(response+1)~explanatory,poissondata) > model4=glm(response~explanatory,poissondata,family=poisson) > model5=glm(response~explanatory,poissondata,family=quasipoisson) > model6=glm.nb(response~explanatory,poissondata) > model7=glm(response~explanatory,quasi(variance="mu",link="identity")) > > > plot(explanatory,response,pch=16) > lines(explanatory,predict(model1,explanatory=explanatory)) > lines(explanatory,(predict(model2,explanatory=explanatory))^2-0.5,lty=2) > lines(explanatory,exp(predict(model3,explanatory=explanatory))-1,lty=3) > lines(explanatory,exp(predict(model5,explanatory=explanatory)),lty=1,col="red") > > lines(explanatory,predict(model6,explanatory=explanatory,type="response"),lty=1,col="blue") > > lines(explanatory,predict(model7,explanatory=explanatory,type="response"),lty=1,col="green") > > > ## > > The only model that performs equally well is model7. > > How would you deal with this kind of analysis? What would be your > recommendation to the students, given the fact that most of the standard > glm models obviously don?t seem to produce good fits here? > > Many thanks and best wishes > Christoph > > (using R 2.8.0 on Windows XP) >
Any good reason that you're not transforming both sides when transforming? and that you're not looking at model8 <- glm(response~log(explanatory),poissondata,family=poisson) (etc.) ?? BTW, your predict call seems to be missing data.frame() around "explanatory=explanatory". The predict() methods do not have an argument called "explanatory", so this is just ignored (a buglet if you ask me). -- O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ R-help@r-project.org 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.
