Re: [R] SEM error

From: John Fox <jfox_at_mcmaster.ca>
Date: Tue, 08 Mar 2011 08:45:59 -0500

Dear Kesinee,

> -----Original Message-----
> From: r-help-bounces_at_r-project.org [mailto:r-help-bounces_at_r-project.org]
> On Behalf Of Kes
> Sent: March-07-11 9:51 PM
> To: r-help_at_r-project.org
> Subject: [R] SEM error
>
> Dear All,
> I am new for R and SEM. I try to fit the model with Y (ordinal outcome),
> X
> (4 categorical data), M1-M3 (continuous), and 2 covariates (Age&sex) as
> a diagram.
> library(polycor)
> model.ly <-specify.model()
> 1: x -> m1, gam11, NA
> 2: x -> m2, gam12, NA
> 3: x -> m3, gam13, NA
> 4: age -> m1, gam14, NA
> 5: age -> m2, gam15, NA
> 6: age -> m3, gam16, NA
> 7: sex -> m1, gam17, NA
> 8: sex -> m2, gam18, NA
> 9: sex -> m3, gam19, NA
> 10: x -> y, gam20, NA
> 11: m1 -> y, gam21, NA
> 12: m2 -> y, gam22, NA
> 13: m3 -> y, gam23, NA
> 14: age -> y, gam24, NA
> 15: sex -> y, gam25, NA,
> 16: m1 <->m1, psi11, NA
> 17: m2 <-> m2, psi12, NA
> 18: m3 <-> m3, psi13, NA
> 19: m1 <-> m2, psi21, NA
> 20: m1 <->m3, psi22, NA
> 21: m2 <-> m3, psi23, NA
> 22: Y <-> Y, psi24, NA,
>
> hcor <-function(ly) hetcor(ly, std.err=FALSE)$correlations R.ly <-
> hcor(ly) sem.ly <- sem(model.ly, R.ly, N=174)
>
> Error in sem.default(ram=ram, S=S, N=N,…………………………) The model has
> negative degree of freedom = -12
>
> First, I do not know what the mistake is. Second, is this correctly
> modeling my diagram? Any suggestions would be appreciated.

First, if x isn't ordinal, you have to create dummy regressors to represent it. Second, your model has no variances or covariances for the exogenous variables (which you could handle compactly with the fixed.x argument to sem). Third, I notice that you use both y and Y in the model. Since Y probably doesn't exist in the input data, sem() treats it as a latent variable. This probably produces the negative df, though I don't know off hand why; had you spelled y correctly, the df should be positive, although, as I've mentioned, the model makes no sense. Finally, this model has a block-recursive structure, and an alternative would be to fit each equation in the model as an appropriate regression model.

I hope this helps,
 John



John Fox
Senator William McMaster
  Professor of Social Statistics
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
http://socserv.mcmaster.ca/jfox

>
> Thank you,
> Kesinee
> http://r.789695.n4.nabble.com/file/n3340497/path.gif.png
>
> --
> View this message in context: http://r.789695.n4.nabble.com/SEM-error-
> tp3340497p3340497.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help_at_r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> and provide commented, minimal, self-contained, reproducible code.



R-help_at_r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Tue 08 Mar 2011 - 14:02:32 GMT

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.2.0, at Tue 08 Mar 2011 - 14:10:20 GMT.

Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help. Please read the posting guide before posting to the list.

list of date sections of archive