variance/mean

Inline Below.

-- Bert 


Bert Gunter
Genentech Nonclinical Biostatistics
650-467-7374

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
Behalf Of Wacek Kusnierczyk
Sent: Sunday, March 22, 2009 2:16 AM
To: rkevinburton at charter.net
Cc: r-help at r-project.org
Subject: Re: [R] variance/mean

At the risk of appearing ignorant why is the folowing true?

o <- cbind(rep(1,3),rep(2,3),rep(3,3))
var(o)
     [,1] [,2] [,3]
[1,]    0    0    0
[2,]    0    0    0
[3,]    0    0    0

and

mean(o)
[1] 2

How do I get mean to return an array similar to var? I would expect in the

above example a vector of length 3 {1,2,3}.

You said:

"you may well be ignorant about how var works with matrices, but this
does not mean it's your fault.  the documentation is typically cryptical."


-- How so? ?var clearly states:

" ... If x and y are matrices then the covariances (or correlations) between
the columns of x and the columns of y are computed. "

and the Arguments section says:

x a numeric vector, matrix or data frame. 
y NULL (default) or a vector, matrix or data frame with compatible
dimensions to x. The default is equivalent to y = x (but more efficient). 


This is as clear as I would know how to state. I think "...typically
cryptical" is a canard and most unfair.

-- Bert