R help - Oct 2010 - hclust with method = “ward”

The clustering function hclust has a method = "ward?, and apparently many
people use that option. However, the Ward method seems to minimize an
increase in the error sums of squares, which are calculated with respect to
the cluster mean. However, hclust has only a dissimilarity matrix as an
input. With this information, one cannot calculate the cluster means, and
consequently cannot use the classic form of the Ward method. My guess is
that the function uses a modified version of Ward, but I could not find any
information about such a modification. I?m wondering if anybody could
explain what is going on here.

Thanks,

Peter

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Apparently, the same issue exists in SAS, where there is an option to run the
Ward algorithm based only on the distance matrix. Perhaps, a SAS user could
confirm that or even check with SAS. 

Peter

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