From: Sunil W <sunnyw_at_hotmail.com>

Date: Mon 05 Dec 2005 - 03:02:48 EST

Dear Sunil,

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Mon Dec 05 03:52:18 2005

Date: Mon 05 Dec 2005 - 03:02:48 EST

Hi John

Thanks a lot for the reply.

Could you suggest how I can correct this problem? I tried using a correlation matrix instead of raw moments, but still got the same error. I also fixed parameters v1,v2,v3,a1 at 1; then it gave me the error that the system is exatly singular.

To answer the points that you raised:

- x1-x6 are not causes; they are just indicatiors. Does that change my model?
- Is the size of the dataset going to be a real limitation? How can I take care of that?

Would appreciate your reply,

Thanks

Sunil

Dear Sunil,

There are 7*8/2 = 28 raw moments among the 7 observed variables. Of these, 6*7/2 = 21 are used for the moments among the 6 fixed-exogenous variables, leaving 28 - 21 = 7 df. You model has 11 free parameters. So df for the model = 11 - 7 = -4.

Some additional comments:

If you're using raw moments, why isn't there a constant variable in the model?

Do you really intend x1 -- x6 to be causes, rather than indicators, of m1 and m2?

Why are there no normalizing constraints on the latent variables?

Do you really want to fit a model like this to so small a data set?

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@stat.math.ethz.ch
**>[mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of Sunil W
**>Sent: Wednesday, November 30, 2005 10:08 PM
**>To: r-help@stat.math.ethz.ch
**>Subject: [R] Error in structural equation model - "The model
**>has negativedegrees of freedom"
**>
**>Hi
**>
**>I am running a structural equation model with R using the sem
**>command; am getting the following error:
**>
**>"Error in sem.default : The model has negative degrees of
**>freedom = -4"
**>
**>
**>My model is as follows:
**>
**>s_model = specify.model()
**>x1->m1, b1,NA
**>x2->m1, b2,NA
**>x3->m2, b3,NA
**>x4->m2, b4,NA
**>x5->m2, b5,NA
**>x6->m2, b6,NA
**>m1->y, a1,NA
**>m2->y, a2,NA
**>m1<->m1, v1,NA
**>m2<->m2, v2,NA
**>y<->y, v3,NA
**>
**>
**>x1-x6 are observed independent variables, m1 and m2 are the
**>latent variables and y is the observed dependent variable. I
**>use the raw.moments command for calculating the covariance
**>matrix, based on a data with 147 observations.
**>
**>The command that I use is as follows:
**>
**>s = sem(s_model,S=R,obs.variables=colnames(R),
**>fixed.x=c('x1','x2','x3','x4','x5','x6'), raw=TRUE)
**>
**>
**>I would appreciate any help on this; I am new to structural
**>equation models
**>and realize that I may be making a silly error.
**>
**>Thanks
**>Sunil
**>
**>______________________________________________
**>R-help@stat.math.ethz.ch mailing list
**>https://stat.ethz.ch/mailman/listinfo/r-help
**>PLEASE do read the posting guide!
**>http://www.R-project.org/posting-guide.html
*

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Mon Dec 05 03:52:18 2005

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