# RE: [R] Structural equation models with R

From: John Fox <jfox_at_mcmaster.ca>
Date: Fri 11 Mar 2005 - 01:22:04 EST

Dear Andre,

> -----Original Message-----
> From: André TavaresCorrêa Dias [mailto:atcdias@biologia.ufrj.br]
> Sent: Thursday, March 10, 2005 6:14 AM
> To: John Fox
> Cc: r-help@stat.math.ethz.ch
> Subject: RE: [R] Structural equation models with R
>
> Dear John,
> Thanks very much! I don’t know how I didn’t see this before.
> I checked a thousand of times…

(Recall that there was a typo in Andre's model specification.)

> There is one more thing that I
> can not understand. What are the possible reasons for
> problems on calculation of confidence interval for RMSEA? For
> the same model I sent before (after correction of the
> variable name) the lower CI output was NA:
>
> Model Chisquare = 10.824 Df = 13 Pr(>Chisq) = 0.62558
> Goodness-of-fit index = 0.91656
> Adjusted goodness-of-fit index = 0.76895
> RMSEA index = 0 90 % CI: (NA, 0.15652)
> BIC = -60.425
>
> I have other models with the same behavior, even when the
> estimative of RMSEA is different from zero.
>
> Model Chisquare = 15.165 Df = 14 Pr(>Chisq) = 0.367
> Goodness-of-fit index = 0.89038
> Adjusted goodness-of-fit index = 0.71811
> RMSEA index = 0.053558 90 % CI: (NA, 0.19141)
> BIC = -61.564
>

The method used to get the confidence interval (from Browne and Du Toit, Multivariate Behavioral Research, 1992, 27:269-300) can produce a lower bound above the RMSEA estimate or an upper bound below the estimate; when this happens, the bound is set to NA. One of the nice things about R is that you can look at the code for a function -- in this case, summary.sem() -- to see exactly what it does. Take a look at how RMSEA.L and RMSEA.U are computed.

Regards,
John

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