Dear Brad, It's impossible to know from the information given whether the model is identified or not. Including a reproducible example of your problem, as the r-help posting guide asks, is a good idea.
Assuming that OCB is an observed variable, however, the model is identified, but implausible. That is, you've specified that the three latent exogenous variables, STA, SA, DA, are uncorrelated. That likely would produce difficulties in maximizing the likelihood, which could explain the error you encountered. Again, without a reproducible example, one can only guess. Some other points: (1) It's generally easier to use specifyEquations() in preference to specifyModel() to define the model. See ?specifyEquations. (2) Even if you use specifyModel(), you need not supply error-variance parameters for endogenous variables; these will be added to the model by default. If you look closely, you'll see that this was done for OCB, for which you omitted the error-variance parameter. See ?specifyModel. (3) You're better off letting sem() compute the observed-variable covariance matrix rather than supplying it as an argument. Among other things, that will allow you to compute robust coefficient standard errors and tests on the fitted model. See ?sem and <http://socserv.mcmaster.ca/jfox/Books/Companion/appendix/Appendix-SEMs.pdf> . I hope this helps, John ----------------------------------------------- John Fox McMaster University Hamilton, Ontario, Canada http://socserv.socsci.mcmaster.ca/jfox/ > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-bounces@r- > project.org] On Behalf Of Yumeng Yue > Sent: Sunday, September 15, 2013 8:31 AM > To: r-help@r-project.org > Subject: [R] Failed path analysis using sem package > > Dear R users: > > I am trying to run a path analysis using sem package in R. But I have > encountered one problem, below is my code: > > SEMEX<-read.csv("D:/Documents and Settings/z3409964/Desktop/Hospital > 1.csv") > library(sem) > cov.matrixSEMEX<-cov(na.omit(SEMEX)) > > SEMEX<-specifyModel() > CWB->CWB13,NA,1 > CWB->CWB23,deviance1 > CWB->CWB33,deviance2 > CWB->CWB43,deviance3 > STA->STA13,NA,1 > STA->STA23,citizenship1 > STA->STA33,citizenship2 > STA->STA43,citizenship3 > STA->STA53,citizenship4 > SA->SA13,NA,1 > SA->SA23,surface1 > SA->SA33,surface2 > DA->DA13,NA,1 > DA->DA23,deep1 > DA->DA33,deep2 > NA->NA13,NA,1 > NA->NA23,negative1 > NA->NA33,negative2 > PA->PA13,NA,1 > PA->PA23,positive1 > PA->PA33,positive2 > CWB13<->CWB13,error1 > CWB23<->CWB23,error2 > CWB33<->CWB33,error3 > CWB43<->CWB43,error4 > STA13<->STA13,error5 > STA23<->STA23,error6 > STA33<->STA33,error7 > STA43<->STA43,error8 > STA53<->STA53,error9 > SA13<->SA13,error10 > SA23<->SA23,error11 > SA33<->SA33,error12 > DA13<->DA13,error13 > DA23<->DA23,error14 > DA33<->DA33,error15 > NA13<->NA13,error16 > NA23<->NA23,error17 > NA33<->NA33,error18 > PA13<->PA13,error19 > PA23<->PA23,error20 > PA33<->PA33,error21 > CWB<->CWB,var1 > STA<->STA,var2 > SA<->SA,var3 > DA<->DA,var4 > PA<->PA,var5 > NA<->NA,var6 > SA->NA,beta1 > NA->CWB,beta2 > DA->PA,beta3 > PA->OCB,beta4 > SA->PA,beta5 > DA->NA,beta6 > OCB<->CWB,cov2 > > SEMEX<-sem(SEMEX,cov.matrixSEMEX, nrow(SEMEX)) > summary(SEMEX,fit.indices=c("CFI","GFI","AGFI","RMSEA")) > > Yet the output shows the following: > > Error in summary.objectiveML(SEMEX, fit.indices = c("CFI", "GFI", > "AGFI", : > coefficient covariances cannot be computed > In addition: Warning message: > In vcov.sem(object, robust = robust, analytic = analytic.se) : > singular Hessian: model is probably underidentified. > > Can you help figure out what is going on? Thank you very much > > Brad > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org 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. ______________________________________________ R-help@r-project.org 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.
