MANOVA polynomial contrasts

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?

Y <- matrix(rnorm(300), 100, 3)
colnames(Y) <- c("y1", "y2", "y3")
f <- ordered(sample(letters[1:4], 100, replace=TRUE))
(mod <- lm(Y ~ f))

Anova(mod)

linearHypothesis(mod, "f.L")

A whole example would be very useful.

Thank you very much in advance!

Best regards,
Gian Mauro Manzoni



2012/7/25 John Fox <jfox at 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.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]]

--
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]]

-- 
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