In general, a "main effects design" need not be orthogonal -- the main effects merely need to be estimable. The trick is to estimate them with good efficiency, etc. I think you need to consult a local statistician for help to understand what these statistical concepts mean.
In your example you could cross the 2^(3-1) with the 3^(3-1) to produce an orthogonal design to estimate main effects. But of course that's 72 runs, which I don't think you would consider "small." As a previous poster commented, there are orthogonal mixed level arrays ("Addleman", "Kempthorne" "Youden" -designs are a couple of phrases to try googling on) which stem from the 1960's. I doubt that, in general, they would satisfy your needs. I have not used the AlgDesign package myself. I suggest you direct questions about it to the author/maintainer, Bob Wheeler. -- Bert Gunter Genentech Non-Clinical Statistics South San Francisco, CA "The business of the statistician is to catalyze the scientific learning process." - George E. P. Box > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of > [EMAIL PROTECTED] > Sent: Monday, January 23, 2006 12:20 PM > To: Berton Gunter; [EMAIL PROTECTED]; > r-help@stat.math.ethz.ch > Subject: [R] R: fractional factorial design in R > > > Yes, you're right. For, say, a 3 x 5 design, one can do > this in as few as > 7 > runs -- but only in general by some version of > one-factor-at-a-time (OFAT) > designs, which are inefficient. It is easy, via, say > model.matrix() to > write a general function to produce these. But I think it's a > bad idea; more > efiicient algorithmic designs are better, IMO, which is why I > suggested > AlgDesign. You and others are free to disagree, of course. > > Hi Bert, > thanks for your suggestion. > However, let us say that i need a 2x2x2x3x3x3 design, which > should not be > too hard. > I've loaded AlgDesign, and i am aware now that gen.factorial > allows me to > create a full desing. But how to create a main-effects-only > factorial design > (orthogonal)? > I am still not able to produce what i need. The function > model.matrix.formula is not very clear... :( > > Could you please indicate which syntax should i use? I'd > really appreciate > your help. > > Thanks in advance, > > Roberto Furlan > University of Turin, Italy > > > ---------------------------------------- > La mia Cartella di Posta in Arrivo è protetta con SPAMfighter > 188 messaggi contenenti spam sono stati bloccati con successo. > Scarica gratuitamente SPAMfighter! > > ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html