Dear Ehlers, thanks for your message. Following the example on stepAIC and Venebles & Ripleys book, it seems that update rearranges the terms. I didnt understand how to indicate the formula in the function.
I have the initial model U ~ var1+var2+var3+var4 (family Gaussian). I want first to select the terms, putting main effects first. If I just write step(model) it will take out no significant variables (lets say var2) and will give U ~ var1+var3+var4. Supposing the var4 have the strongest effect upon U, followed by var1 and var3, I would like to have the out put U ~ var4+var1+var2. Is it possible to do so? Thanks again. Antonio Olinto Biologist Sao Paulo Fisheries Institute PS. Unfortunately I dont have Regression Modeling Strategies around here Citando P Ehlers <[EMAIL PROTECTED]>: > > Antonio Olinto wrote: > > > Hello, > > > > I have a doubt in using the function step (step wise) to select glm > models. > > > > Usually I apply the gamma distribution to analyze fishery data. To select > the > > terms I use a routine where I first compare single term models to the null > model > > (eg. U~1 vs. U~depth; U~1 vs. U~latitude; etc. where U= abundance) and, > by > > means of the result given by a likelihook function applied for each > comparison, > > I select the strongest effect, lets say depth. Then I run a new step > > comparing the U~depth vs. U~depth+latitude; U~depth vs. U~depth+... etc. > Making > > this way I put the terms in magnitude order. > > > > I tried to make a gaussian model using the step(glm.model) function to > select > > the terms but I saw that in the output table given by anova(glm.model) the > > selected terms kept the original order. > > > > Is it possible to have the terms in the model rearranged, as in my > example? > > > > Thanks for any help. I read Chambers and Hasties Statistical Models in > S, > > Venables and Ripley Modern Applied Statistics and, of course, R help but > I > > couldnt get the trick. > > > > I don't know if this will get you there, but > > 1. I would use stepAIC in package MASS; > 2. set argument trace = TRUE; > 3. think very hard about the interpretation of the model; > 4. read also Frank Harrell's "Regression Modeling Strategies". > > Peter > > > > Antonio > > > > -- > > Biologist > > Sao Paulo Fisheries Institute > > > > > > > > ------------------------------------------------- > > WebMail Bignet - O seu provedor do litoral > > www.bignet.com.br > > > > ______________________________________________ > > [email protected] mailing list > > https://stat.ethz.ch/mailman/listinfo/r-help > > PLEASE do read the posting guide! > http://www.R-project.org/posting-guide.html > > -- > Peter Ehlers > Department of Mathematics and Statistics > University of Calgary, 2500 University Dr. NW ph: 403-220-3936 > Calgary, Alberta T2N 1N4, CANADA fax: 403-282-5150 > > > ------------------------------------------------- WebMail Bignet - O seu provedor do litoral www.bignet.com.br ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
