[R] Mahalanobis

[Alberto Murta]

Dear all

Why isn'it possible to calculate Mahalanobis distances with R for a matrix 
with 1 row (observations) more than the number of columns (variables)?

> mydata <- matrix(runif(12,-5,5), 4, 3)
> mahalanobis(x=mydata, center=apply(mydata,2,mean), cov=var(mydata))
[1] 2.25 2.25 2.25 2.25

> mydata <- matrix(runif(420,-5,5), 21, 20)
> mahalanobis(x=mydata, center=apply(mydata,2,mean), cov=var(mydata))
 [1] 19.04762 19.04762 19.04762 19.04762 19.04762 19.04762 19.04762 19.04762 
19.04762 19.04762 19.04762 19.04762
[13] 19.04762 19.04762 19.04762 19.04762 19.04762 19.04762 19.04762 19.04762 
19.04762

> mydata <- matrix(runif(132,-5,5), 12, 11)
> mahalanobis(x=mydata, center=apply(mydata,2,mean), cov=var(mydata))
 [1] 10.08333 10.08333 10.08333 10.08333 10.08333 10.08333 10.08333 10.08333 
10.08333 10.08333 10.08333 10.08333

Thanks in advance

Alberto Murta

> version
         _                
platform i686-pc-linux-gnu
arch     i686             
os       linux-gnu        
system   i686, linux-gnu  
status                    
major    1                
minor    8.1              
year     2003             
month    11               
day      21               
language R                

-- 
                                         Alberto G. Murta
Institute for Agriculture and Fisheries Research (INIAP-IPIMAR) 
Av. Brasilia, 1449-006 Lisboa, Portugal | Phone: +351 213027062
Fax:+351 213015948 | http://ipimar-iniap.ipimar.pt/pelagicos/