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RE: st: RE: Cluster analysis on survey data


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: Cluster analysis on survey data
Date   Fri, 29 Aug 2008 17:07:47 +0100

Thanks for this detailed reply. 

The characterization that you were given that cluster analysis looks at
medians is not very illuminating. Cluster analysis is not a single
method, or even a family of methods, but a clutch of loosely similar
techniques. How clusters are defined, whether by using medians in some
sense or by some other way of summarizing, is up to the user, and there
are lots and lots of ways to do it, which is part of the charm, or some
say the capriciousness, of the field. 

Nick 
n.j.cox@durham.ac.uk 

Tullar, Jessica M

First thank you both for responding.

To answer your questions... 

As far as the method to describe the kinds of people that report medical
debt and medical bankruptcy... It was explained to me that regression
(logit with survey weights was my first choice for how to answer this
question, mlogit is an even better idea) looked at means while cluster
analysis (the alternative method) looked at "medians". The implication
of compared to... did not come into the discussion but makes a good
point which I will bring back to my group.

As to the other suggestion that cluster analysis did not need survey
weights. You are correct that if all we are concerned about is
description then the comparison of closeness of observations then
whether they represent more or less individuals doesn't seem
particularly concerning. However, my concerns arose from reading the
chapter from Reading and Understanding More Multivariate Statistics
(Grimm and Yarnold 2000) they discuss the importance of the
representativeness of your sample (survey data would not be
representative unless weighted) and that some cluster analysis methods
are interested in equal sized groups (again dependent upon analyzing a
true representative sample). However if we don't use methods that look
at the size of groups and focus on the distance between observations
then describing those groups using cluster analysis without weights does
not seem too inappropriate as long as the focus and description are
clear about the non-representativeness.

As an aside, sorry about the unexplained reference in the original
request. BRFSS is the Behavioral Risk Factor Surveillance System, a
large ongoing telephone survey run through the U.S. Centers for Disease
Control.


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