Dear Prof. John Fox,
thank you very much for your suggestions.
However, I still do not know how to use the contrasts after generating them.
Once I generate the matrix with the polynomial contrasts, what are the
following steps toward the statistical test?
A whole example would be very useful.
Thank you very much in advance!
Best regards,
Gian Mauro Manzoni
2012/7/25 John Fox <j...@mcmaster.ca>
> 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.manz...@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 <538...@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.manz...@auxologico.it
> > >> cell. phone +39 338 4451207
> > >> Tel. +39 0323 514278
> > >>
> > >> [[alternative HTML version deleted]]
> > >>
> > >> ______________________________________________
> > >> 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.
> > >>
> > >
> > >
> > > --
> > > Gregory (Greg) L. Snow Ph.D.
> > > 538...@gmail.com
> > >
> >
> >
> >
> > --
> > Dr. Gian Mauro Manzoni
> > PhD, PsyD
> > Psychology Research Laboratory
> > San Giuseppe Hospital
> > Istituto Auxologico Italiano
> > Verbania - Italy
> > e-mail: gm.manz...@auxologico.it
> > cell. phone +39 338 4451207
> > Tel. +39 0323 514278
> >
> > [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > 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.
>
>
>
--
Dr. Gian Mauro Manzoni
PhD, PsyD
Psychology Research Laboratory
San Giuseppe Hospital
Istituto Auxologico Italiano
Verbania - Italy
e-mail: gm.manz...@auxologico.it
cell. phone +39 338 4451207
Tel. +39 0323 514278
[[alternative HTML version deleted]]
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