[R] About help on 'mahalanobis'
Henrik Bengtsson
hb at maths.lth.se
Wed Dec 14 07:05:32 CET 2005
Hi,
help on 'mahalanobis' (in the stats package in Rv2.2.0) now says:
"Description:
Returns the Mahalanobis distance of all rows in 'x' and the vector
mu='center' with respect to Sigma='cov'. This is (for vector 'x')
defined as
D^2 = (x - mu)' Sigma^{-1} (x - mu)"
It does return D^2 as written. However, would the text be more clear if
it says "Returns the _squared_ Mahalanobis distance D^2..." instead? If
so, then text in the example code, e.g. "##- Here, D^2 = usual Euclidean
distances" and the title of the first plot will also have to be updated.
Compare this with what dist() in the same package returns. When asking
for the Equlidean distance (matrix) between rows in a matrix, we get D
not D^2, e.g. dist(c(1,3)) == 2.
Cheers
Henrik
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