isoMDS

I don't understand.
If isoMDS does not work with distances, why does the help for isoMDS
indicate that the "Data are assumed to be dissimilarities or relative
distances" ? 
Equally confusing is the loose use of the terms dissimilarities and
distances in the literature. As you point out in your book "Distances are
often called disimilarities". 

-----Original Message-----
From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk] 
Sent: September 8, 2004 11:58 AM
To: Hanke, Alex
Cc: 'Doran, Harold'; 'r-help at stat.math.ethz.ch'
Subject: RE: [R] isoMDS

Distances cannot always be constructed from similarities. This can be done
only if the matrix of similarities is nonnegative definite. With the
nonnegative definite condition, and with the maximum similarity scaled so
that s_ii=1, d_ik=(2*(1-s_ik))^-.5

But isoMDDS works with dissimilarities not distances.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595