Message-ID: <17311.59238.845567.164212@stat.math.ethz.ch>
Date: 2005-12-14T09:35:34Z
From: Martin Maechler
Subject: About help on 'mahalanobis'
In-Reply-To: <439FB62C.9040009@maths.lth.se>
>>>>> "HenrikB" == Henrik Bengtsson <hb at maths.lth.se>
>>>>> on Wed, 14 Dec 2005 17:05:32 +1100 writes:
HenrikB> Hi,
HenrikB> help on 'mahalanobis' (in the stats package in Rv2.2.0) now says:
HenrikB> "Description:
HenrikB> Returns the Mahalanobis distance of all rows in 'x' and the vector
HenrikB> mu='center' with respect to Sigma='cov'. This is (for vector 'x')
HenrikB> defined as
HenrikB> D^2 = (x - mu)' Sigma^{-1} (x - mu)"
HenrikB> It does return D^2 as written. However, would the
HenrikB> text be more clear if it says "Returns the
HenrikB> _squared_ Mahalanobis distance D^2..." instead? If
HenrikB> so, then text in the example code, e.g. "##- Here,
HenrikB> D^2 = usual Euclidean distances" and the title of
HenrikB> the first plot will also have to be updated.
Indeed, thank you!
Committed {even to R-beta}.
HenrikB> Compare this with what dist() in the same package
HenrikB> returns. When asking for the Equlidean distance
HenrikB> (matrix) between rows in a matrix, we get D not
HenrikB> D^2, e.g. dist(c(1,3)) == 2.