Those are contrasts treating the interaction as a set of four treatments, not an interaction between two factors. So you would need to set contrasts on the factor ab <- a:b (outside a formula), with formula a+b+ab.
On Fri, 5 Mar 2004, Christophe Pallier wrote: > > > Prof Brian Ripley wrote: > > >On Fri, 5 Mar 2004, pallier wrote: > > > >... > > > > > > > >>Actually, the different types of main effects defined above just > >>correspond to different > >>contrasts on the cell means. So if there is an easy solution to compute > >>arbitrary contrasts > >>on the cell means in a factorial design, this could an approach to this > >>question. (Anyone?) > >> > >> > > > >There are at least three such ways. ?contrasts (for the assignment > >function contrasts<-) and ?C, as well as the contrasts= argument to aov > >(the function you were discussing ...). > > > > > Thanks. > I know the existence of 'contrasts' and I read the section about > contrasts matrix in your book (MASS 3rd edition), as well as > in the R online documentation, but I probably do not understand them > well: It still escapes me how to proceed to compute > "arbitrary" contrasts, such as, say: > > a1b1 a1b2 a2b1 a2b2 > 1 1 -1 -1 > > a1b1 a1b2 a2b1 a2b2 > .5 .5 -1 0 > > in a model "x~ a * b" where a and b are two binary factors. > > (the contrasts should be on the cell means, ignoring the sample size of > subgroups. I know how to compute the size of the contrasts from the > table of means returned by tapply, but I whould also need the associated > MSE). > > Sorry if the solution is obvious. > > Christophe Pallier > > > > > -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
