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st: Re: Clustermat puzzle


From   brendan.halpin@ul.ie (Brendan Halpin)
To   statalist@hsphsun2.harvard.edu
Subject   st: Re: Clustermat puzzle
Date   Sun, 25 Mar 2012 15:55:24 +0100

Following up to myself:
On Sat, Mar 24 2012, Brendan Halpin wrote:

> I have a small matrix of pairwise distances (all integers) that I'm
> passing to clustermat (Ward's method). I notice that if I scale the
> distances by a constant, I get different results. 

On mature reflection, this is clearly a precision issue. What must be
happenning is that at some stage in the clustering, when the candidates
for the next agglomeration are considered, what is a clear tie with the
integer distance matrix, results in a miniscule difference with the
scaled distance matrix. Using the integer matrix, the next agglomeration
is picked from among the ties (at random, or arbitrarily); using the
scaled matrix, the next agglomeration is determinately the one that has
just (illegitately) shaded the comparison.

Is there any way of controlling the tolerance used in the test for ties?
As far as I can see, the documentation for -cluster- and -clustermat-
doesn't discuss ties or tolerance. 

Regards,

Brendan
-- 
Brendan Halpin,   Department of Sociology,   University of Limerick,   Ireland
Tel: w +353-61-213147  f +353-61-202569  h +353-61-338562;  Room F1-009 x 3147
mailto:brendan.halpin@ul.ie    ULSociology on Facebook: http://on.fb.me/fjIK9t
http://teaching.sociology.ul.ie/bhalpin/wordpress         twitter:@ULSociology
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