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Re: st: Polychoric and missing values in correlation matrix


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Polychoric and missing values in correlation matrix
Date   Thu, 9 Apr 2009 15:43:18 +0000 (GMT)

--- On Thu, 9/4/09, Juan Julio Gutierrez <juanju@yahoo.com> wrote:
> I am using the polychroric command to calculate the
> correlation matrix of a set of 17 variables ( 8 binary 0-/1
> and 9 metric). 
> 
> After lenghty calculations STATA shows a matrix  with
> missing values in the matrix, mostly between metric and
> binary variables. 4 metric variables have missing values
> correlations with binary variables. 
> 
> The results window show these messages. In red: 
> 
> could not calculate numerical derivatives
<snip>

As -polychoric- is not an official Stata command, please 
report where you got it from.

The problem does not appear to be missing values but 
that your model did not converge. Some things that might
cause this is when any of your binary variables have 
very few ones or zeros, or when there are near perfect
dependencies between your variables. 

Hope this helps,
Maarten

-----------------------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany

http://home.fsw.vu.nl/m.buis/
-----------------------------------------


      

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