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.
