[R] No fit statistics for some models using sem
Ista Zahn
istazahn at gmail.com
Wed Mar 7 18:59:34 CET 2007
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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