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Re: Cluster analysis [message #41889 is a reply to message #11153]

Sun, 05 December 2004 06:43

George N. White III is currently offline

George N. White III
Messages: 56
Registered: September 2000

Member

On Fri, 3 Dec 2004, Chris Jacobsen wrote:

> In our work we started out using the "stock" IDL
> cluster routine but we have added to it a bit.
> Still, we have not changed the basic algorithm.
> We've found that preparation of the data can
> make a big difference. If the variation in
> variable X is 100 times bigger than the variation
> in variable Y, then the clustering (which looks
> at simple Euclidian distance) will not see the
> variation in Y very well. One approach is
> to subtract the mean of each variable, and
> apply a scale factor to the data in variable
> Y so that it is spread out over the same distance
> as in variable X.

Sound advice. You should also consider whether a
non-linear transform (e.g. alog()) should be applied
to some variables. Many people overlook the rank
transform, which gives you a distance measure that
is essentially the number of observations with values
between the two points. This is a way to make sense
of comparisons between different physical quantities.

--
George N. White III <aa056@chebucto.ns.ca>

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[Message index]

		Cluster analysis By: mph410 on Fri, 27 February 1998 00:00
		Re: Cluster analysis By: David Foster on Mon, 09 March 1998 00:00
		Re: Cluster analysis By: Chris[2] on Tue, 07 December 2004 11:32
		Re: Cluster analysis By: Harald Frey on Mon, 06 December 2004 14:21
		Re: Cluster analysis By: George N. White III on Sun, 05 December 2004 06:43
		Re: Cluster analysis By: Chris.Jacobsen on Fri, 03 December 2004 15:30
		Re: Cluster analysis By: James Kuyper on Fri, 03 December 2004 06:44

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