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