[R] MANOVA polynomial contrasts

[John Fox]

Dear Gian,

How contrasts are created by default is controlled by the contrasts option:

> getOption("contrasts")
        unordered           ordered 
"contr.treatment"      "contr.poly" 

So, unless you've changed this option, contr.poly() will be used to generate orthogonal polynomial contrasts for an ordered factor, and you therefore need do nothing special to get this result. For example:

> (f <- ordered(sample(letters[1:3], 10, replace=TRUE)))
 [1] c c a a c c b c a c
Levels: a < b < c

> round(contrasts(f), 4)
          .L      .Q
[1,] -0.7071  0.4082
[2,]  0.0000 -0.8165
[3,]  0.7071  0.4082

For more information, see section 11 on statistical models in the manual "An Introduction to R," which is part of the standard R distribution, and in particular sections 11.1 and 11.1.1.

I hope that this clarifies the issue.

Best,
 John

------------------------------------------------
John Fox
Sen. William McMaster Prof. of Social Statistics
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
http://socserv.mcmaster.ca/jfox/

On Wed, 25 Jul 2012 11:58:30 +0200
 "Manzoni, GianMauro" <gm.manzoni at auxologico.it> wrote:
> Dear Greg Snow,
> thank you very much for your suggestions. However, I need an example in
> order to understand fully.
> I was told that, given the ordinal factor, I do not need to specify the
> contr.poly function because R does it automatically.
> However, I don not know if I have to add an argument into the manova/anova
> function or something else.
> Please write me an illustrative example.
> Many thanks.
> 
> Best regards,
> Gian Mauro Manzoni
> 
> 2012/7/25 Greg Snow <538280 at gmail.com>
> 
> > You should not need to write them yourself. Look at the contr.poly
> > function along with the C function (Note uppercase C) or the contrasts
> > function.
> >
> >
> > On Monday, July 23, 2012, Manzoni, GianMauro wrote:
> >
> >> Dear all,
> >> I am quite new to R and I am having trouble writing the polynomial
> >> contrasts for an ordinal factor in MANOVA.
> >> # I have a model such as this
> >> fit<-manova(cbind(Y1,Y2,Y3)~Groups,data=Events) # where groups is an
> >> ordinal factor with 4 levels
> >> # how to set polynomial contrasts for the "Groups" factor ?
> >>
> >> Thank you very much in advance for any help!
> >>
> >> Best regards,
> >> Mauro
> >>
> >> --
> >> Dr. Gian Mauro Manzoni
> >> PhD, PsyD
> >> Psychology Research Laboratory
> >> San Giuseppe Hospital
> >> Istituto Auxologico Italiano
> >> Verbania - Italy
> >> e-mail: gm.manzoni at auxologico.it
> >> cell. phone +39 338 4451207
> >> Tel. +39 0323 514278
> >>
> >>         [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
> >> R-help at 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.
> >>
> >
> >
> > --
> > Gregory (Greg) L. Snow Ph.D.
> > 538280 at gmail.com
> >
> 
> 
> 
> -- 
> Dr. Gian Mauro Manzoni
> PhD, PsyD
> Psychology Research Laboratory
> San Giuseppe Hospital
> Istituto Auxologico Italiano
> Verbania - Italy
> e-mail: gm.manzoni at auxologico.it
> cell. phone +39 338 4451207
> Tel. +39 0323 514278
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at 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.