R help - Apr 2007 - Dealing with large nominal predictor in sem package

Hi,

I am using tsls function from sem package to estimate a model which includes
large number of data. Among its predictors, it has a nominal data which has
about 10 possible values. So I expand this parameter into 9-binary-value
predictors with the coefficient of base value equals 0. I also have another
continuous predictor.

The problem is that, whenever I run the tsls, I will get 'System is
computationally singular' error all the time. I'm wondering if there is
anyway that I can overcome this problem? Please kindly suggest. Thank you so
much in advance.

- adschai

	[[alternative HTML version deleted]]

Dear adschai,

It's not possible to know from your description exactly what you're
doing,
but perhaps the following will help: 

(1) I presume that your nominal variable is exogenous, since otherwise it
wouldn't be sensible to use 2SLS. 

(2) You don't have to make your own dummy regressors for a nominal variable;
just represent it in the model as a factor as you would, e.g., in lm(). 

(3) Do you have at least as many instrumental variables (including the dummy
regressors) as there are structural coefficients to estimate? If not, the
structural equation is underidentified, which will produce the error that
you've encountered.

I hope this helps,
 John

--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox 
-------------------------------- 
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of 
> adschai at optonline.net
> Sent: Sunday, April 08, 2007 11:07 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Dealing with large nominal predictor in sem package
> 
> Hi,
> 
> I am using tsls function from sem package to estimate a model 
> which includes large number of data. Among its predictors, it 
> has a nominal data which has about 10 possible values. So I 
> expand this parameter into 9-binary-value predictors with the 
> coefficient of base value equals 0. I also have another 
> continuous predictor. 
> 
> The problem is that, whenever I run the tsls, I will get 
> 'System is computationally singular' error all the time. I'm 
> wondering if there is anyway that I can overcome this 
> problem? Please kindly suggest. Thank you so much in advance.
> 
> - adschai
> 
> 	[[alternative HTML version deleted]]
> 
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