[R] R: fractional factorial design in R

[Berton Gunter]

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: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of 
> statistical.model at googlemail.com
> Sent: Monday, January 23, 2006 12:20 PM
> To: Berton Gunter; statistical.model at googlemail.com; 
> r-help at 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
> 
> 
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