[R] No fit statistics for some models using sem

[David Barron]

It's not the correlation as such that is the problem; it's because you
only have 10 degrees of freedom "available" with four observed
variables, and you are estimating 10 parameters, which is why you get
a chi square of zero.  When you remove any one free parameter (such as
the correlation), the model becomes identified.

On 07/03/07, Ista Zahn <istazahn at gmail.com> wrote:
> Hi,
>
> New to both R and SEM, so this may be a very simple question. I am
> trying to run a very simple path analysis using the sem package.
> There are 2 exogenous (FARSCH, LOCUS10) and 2 endogenous (T_ATTENT,
> RMTEST) observed variables in the model.  The idea is that T_ATTENT
> mediates the effect of FARSCH and LOCUS10 on RMTEST. The RAM
> specification I used is
>
> FARSCH -> T_ATTENT, y1x1, NA
> LOCUS10 -> T_ATTENT, y1x2, NA
> FARSCH -> RMTEST10, y2x1, NA
> LOCUS10 -> RMTEST10, y2x2, NA
> T_ATTENT -> RMTEST10, y2y1, NA
> FARSCH <-> FARSCH, x1x1, NA
> LOCUS10 <-> LOCUS10, x2x2, NA
> T_ATTENT <-> T_ATTENT, y1y1, NA
> RMTEST10 <-> RMTEST10, y2y2, NA
> LOCUS10 <-> FARSCH, x2x1, NA
>
> This model runs, but using the summary function does not return the
> usual model fit statistics, only the following:
>
> Model Chisquare =  0   Df =  0 Pr(>Chisq) = NA
>   Chisquare (null model) =  8526.8   Df =  6
>   Goodness-of-fit index =  1
>   BIC =  0
>
> If I omit the last line from the RAM specification(i.e., delete
> "LOCUS10 <-> FARSCH, x2x1, NA"), I DO get all the usual statistics:
>
>   Model Chisquare =  1303.7   Df =  1 Pr(>Chisq) = 0
>   Chisquare (null model) =  8526.8   Df =  6
>   Goodness-of-fit index =  0.95864
>   Adjusted goodness-of-fit index =  0.58639
>   RMSEA index =  0.30029   90% CI: (NA, NA)
>   Bentler-Bonnett NFI =  0.84711
>   Tucker-Lewis NNFI =  0.082726
>   Bentler CFI =  0.84712
>   BIC =  1294.1
>
> My understanding is the you should always put in the correlation
> between exogenous predictors, but when I do this I don't get fit
> statistics. Can anyone help me understand what is happening here?
>
> Thank you,
>
> Ista
>
> ______________________________________________
> R-help at 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
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
=================================
David Barron
Said Business School
University of Oxford
Park End Street
Oxford OX1 1HP