Hi, It seems that you have an unbalanced anova case which can be complicated in interpretation. This question has been addressed in the list you can search of previous mails. Basically, for unbalanced data (or unorthorgonal), estimates of SS of a factor depends on other factors. Type I SS (which is the method by aov) is a sequential method. Therefore, order of variables in the model matters. For your situation, you may want to use type II or type III SS (check Anova in car) because they are estimates adjusted for other variables in the model, thus order-independent.
Weidong Gu On Thu, Mar 22, 2012 at 5:06 PM, Stuart Luppescu <s...@ccsr.uchicago.edu> wrote: > Hello, This one is very perplexing. I have teacher observation data, > with factors teacher ID, observer ID, component, grade and subject. When > I do this, > > aov(data=ratings.prin.22, rating ~ obsid.f + tid.f + subject.f + grade.f + > comp.f) > > I get this: > > Terms: > obsid.f tid.f grade.f comp.f Residuals > Sum of Squares 306.23399 221.38173 1.70000 14.52831 279.05780 > Deg. of Freedom 74 87 2 9 1406 > > This looks right. There are about 82 observers so 74 degrees of freedom > for obsid.f is OK. But if I do this (just reordering the terms in the > formula): > > aov(data=ratings.prin.22, rating ~ comp.f + grade.f + subject.f + tid.f + > obsid.f) > > I get this: > > Terms: > comp.f grade.f subject.f tid.f obsid.f Residuals > Sum of Squares 15.0923 23.3968 5.9982 499.3568 0.0000 279.0578 > Deg. of Freedom 9 4 4 152 3 1406 > > Now obsid.f only has 3 degrees of freedom and the sum of squares is > 0.00. Could this be due to the unbalanced design? > > If someone can explain this to me I would be very grateful. > > -- > Stuart Luppescu -=- slu .at. ccsr.uchicago.edu > University of Chicago -=- CCSR > 才文と智奈美の父 -=- Kernel 3.2.1-gentoo-r2 > Tony Plate: There looks to be a typo in the R-exts > manual: [...] Peter Dalgaard: 'svn blame' tells me > that this was Brian's addition in rev.35362 [...] > Brian D. Ripley: I prefer 'svn praise' myself. > Peter Dalgaard: Or 'svn annotate'. I think it > depends on what I'm looking for, plus the risk > that the author (perpetrator, contributor) might > be me... -- Tony Plate, Peter Dalgaard and > Brian D. Ripley (about a typo in the > > ______________________________________________ > 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.