[R] No fit statistics for some models using sem

[Ista Zahn]

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