[R] chi square and mahalanobis

[Mauro A. Cremonini]

Hi all.
I am seeking for a statistical suggestion.

My data set comprises 382 measures each having 169 variables. Each measure
is the outcome of a nuclear magnetic resonance experiment, so each of the
169 points has the same unit.

As I want to do some multivariate calibration using these data, I checked 
whether some multivariate outliers existed. I calculated Mahalanobis
distances and did a 

qqplot(qchisq(ppoints(382),169),XMaha) 

where XMaha contained my Mahalanobis distances. The points on my QQ plot
were nicely aligned onto a y=x line, so no outliers.

For some reasons I need to scale my data set so as that each measure
has the same initial point, i.e. the first point of each measure must be
1. To be clear I did:

x1<-x[,1:169]/x[,1]

where x and x1 are the unscaled and scaled sata sets, respectively.

However, after doing this (and eliminating the first point of each
measure), the points in the new QQ plot - 

qqplot(qchisq(ppoints(382),168),XMahaNew)  - 

were still aligned, although they were no more along the y=x line.

Am I doing something statistically wrong ?

Thank you all in advance.

Mauro





============================================
Mauro Andrea Cremonini, PhD
Department of Food Science
University of Bologna
Via Ravennate 1020 - 47023 - Cesena - Italy
FAX:+39.0547.382348 - Phone:+39.0547.636106
e-mail: mac at foodsci.unibo.it  
http://www.foodsci.unibo.it/cremonini
============================================


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