[R] About help on 'mahalanobis'

Henrik Bengtsson hb at maths.lth.se
Wed Dec 14 07:05:32 CET 2005


help on 'mahalanobis' (in the stats package in Rv2.2.0) now says:


      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.



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