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"polychoric estimates polychoric and polyserial correlations, and
polychoricpca performs the principal component analysis on the resulting
correlation matrix. The current version (1.4) of the routine requires Stata 8.2.

"The polychoric correlation of two ordinal variables is derived as follows.
 Suppose each of the ordinal variables was obtained by categorizing a
normally distributed underlying variable, and those two unobserved variables
follow a bivariate normal distribution. Then the (maximum likelihood)
estimate of that correlation is the polychoric correlation. If each
of the ordinal variables has only two categories, then the correlation
between the two variables is referred to as tetrachoric."

Hope this helps.

--Mark

Quoting Patricia Sourdin <patricia.sourdin@adelaide.edu.au>:

> Hi Statalist
> just a query on -factor-.
> I am trying to construct an index where I have five variables which
> are binary
> indicators.
> I have read somewhere that it is not appropriate to use factor
> analysis if the
> variables are binary.  Can anyone confirm, please?
> Also, would -pca- be an alternative in this case?
> As usual, any help would be appreciated.
> Patricia
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> 



Prof. Mark Schaffer
Director, CERT
Department of Economics
School of Management & Languages
Heriot-Watt University, Edinburgh EH14 4AS
tel +44-131-451-3494 / fax +44-131-451-3008
email: m.e.schaffer@hw.ac.uk
web: http://www.sml.hw.ac.uk/ecomes
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