variable standardization in manova() call

2013/11/6 Michael Friendly <friendly at yorku.ca>:

On 11/4/2013 10:45 AM, Sergio Fonda wrote:

Hi,
I'm not able to get information about the following question:

is the variables standardization a default option in manova() (stats
package)?
Or if you want to compare variables with different units or scales and
rather different variances, you have to previously standardize the
variables ?

If you mean the response variables, manova() does not require equal
variances and does not standardize.


--
Michael Friendly     Email: friendly AT yorku DOT ca
Professor, Psychology Dept. & Chair, Quantitative Methods
York University      Voice: 416 736-2100 x66249 Fax: 416 736-5814
4700 Keele Street    Web:   http://www.datavis.ca
Toronto, ONT  M3J 1P3 CANADA

Hi Michael,
thank you for your reply. However, your remark was not so clear to me
so I attach a short script I tried to launch. The comparison between
results got from MANOVA() call with the non-standardized and
standardized version of the same data frame, convinced me that it is
not necessary to standardize data before calling MANOVA. The only
difference you get is in the residuals values, of course.
Could you kindly confirm my conclusion?

All the best,
Sergio Fonda
______________________
set.seed(1234)
# non- standardized series
x1 =  rnorm(n=10, mean=50, sd=11)
x2 =  rnorm(n=10, mean=93, sd=23)
x1 =  rnorm(n=10, mean=217, sd=52)
fact= rep(1:2,20)
glob1=data.frame(cbind(x1,x2,x3,fact))
fitta1=manova(cbind(x1,x2,x3)~fact, data=glob1)
fitta1.wilks=summary(fitta1, test="Wilks")
summary.aov(fitta1)

#after standardization
x.stand=scale(glob1[,-4])
glob2=data.frame(x.stand,fact)
fitta2=manova(cbind(x1,x2,x3)~fact, data=glob2)
fitta2.wilks=summary(fitta2, test="Wilks")
summary.aov(fitta2)

Sergio Fonda
Associate Professor of Bioengineering
Department of Life Sciences
University of Modena and Reggio Emilia
Modena, Italy