Dear Anita and Joris, Please see <https://stat.ethz.ch/pipermail/r-help/2010-March/230280.html>, posted to r-help in March.
Regards, John -------------------------------- John Fox Senator William McMaster Professor of Social Statistics Department of Sociology McMaster University Hamilton, Ontario, Canada web: socserv.mcmaster.ca/jfox > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On > Behalf Of Anita Narwani > Sent: June-03-10 8:49 PM > To: Joris Meys > Cc: r-help@r-project.org > Subject: Re: [R] Nested ANOVA with covariate using Type III sums of squares > > Yes I understood the strangeness of removing a main effect without > interactions that contain it because I did this during my efforts using > model simplification. I had checked out the link you sent a couple of days > ago. It was useful. So does Type II SS remove both the factor and any > interactions containing it when comparing models? i.e. to test for the main > effect of B you compare A + B + A:B against A? > > On Thu, Jun 3, 2010 at 4:59 PM, Joris Meys <jorism...@gmail.com> wrote: > > > Hi Anita, > > > > I have to correct myself too, I've been rambling a bit. Off course you > > don't delete the variable out of the interaction term when you test the > main > > effect. What I said earlier didn't really make any sense. > > > > That testing a main effect without removing the interaction term is has a > > tricky interpretation. By removing a main effect you test full model A + B > > + A:B against the model A + A:B. If you remove the main effect "Zoop" for > > example, you basically nest Zoop within Diversity and test whether that's > > not worse than the full model. This explains it very well: > > > > https://stat.ethz.ch/pipermail/r-help/2010-March/230280.html > > > > I'd go for type II, but you're free to test any hypothesis you want. > > > > Cheers > > Joris > > > > > > > > On Thu, Jun 3, 2010 at 9:59 PM, Anita Narwani > <anitanarw...@gmail.com>wrote: > > > >> Thanks for your response Joris. > >> > >> I was aware of the potential for aliasing, although I thought that this > >> was only a problem when you have missing cell means. It was interesting to > >> read the vehement argument regarding the Type III sums of squares, and > >> although I knew that there were different positions on the topic, I had no > >> idea how divisive it was. Nevertheless, Type III SS are generally > >> recommended by statistical texts in ecology for my type of experimental > >> design. Interestingly, despite the aliasing, SPSS has no problems > >> calculating Type III SS for this data set. This is simply because I am > >> entering a co-variate, which causes non-orthogonality. I would be happier > >> using R and the default Type I SS, which are the same as the Type III SS > >> anyway when I omit the co-variate of Mean.richness, except that these > >> results are very sensitive to the order in which I add the variables into > >> the model when I do enter the co-variate. I understand that the order is > >> very important relates back to the scientific hypothesis, but I am equally > >> interested in the main effects of Zoop, Diversity, and the nested effect > of > >> Phyto, so entering either of these variables before the other does not > make > >> sense from an ecological perspective, and because the results do change, > the > >> order cannot be ignored from a statistical perspective. > >> Finally, I have tried using the Type II SS and received similar warnings. > >> > >> Do you have a recommendations? > >> Anita. > >> > > > > > > > > -- > > Joris Meys > > Statistical Consultant > > > > > > Ghent University > > Faculty of Bioscience Engineering > > Department of Applied mathematics, biometrics and process control > > > > Coupure Links 653 > > B-9000 Gent > > > > tel : +32 9 264 59 87 > > joris.m...@ugent.be > > ------------------------------- > > Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php > > > > [[alternative HTML version deleted]] > > ______________________________________________ > 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. ______________________________________________ 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.
