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 <[EMAIL PROTECTED]> 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@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 ______________________________________________ R-help@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.