Mplus Users Guide v6.. - Muthén & Muthén

Special Modeling Issuesof the MODEL command in conjunction with the | symbol of theMODEL command and in growth models for censored, categorical, andcount outcomes. If convergence problems arise, information in the errormessages identifies the problematic variable. In addition, the output canbe examined to see the size of the random effect variable variance. If itis close to zero and the random effect variable is a random slope definedusing an ON statement in conjunction with the | symbol, a fixed effectshould be used instead by using a regular ON statement. If it is close tozero and the random effect variable is a growth factor, the growth factorvariance and corresponding covariances should be fixed at zero.CONVERGENCE PROBLEMS SPECIFIC TO MIXTUREMODELSIn mixture models, convergence is determined not only by thederivatives of the loglikelihood but also by the absolute and relativechanges in the loglikelihood and the changes in the class counts.Information about changes in the loglikelihood and the class counts canbe found in TECH8.Even when a mixture model does converge, it is possible to obtain alocal solution. Therefore, it is important to run the model with multiplesets of starting values to guarantee that the best solution is obtained.The best solution is the solution with the largest loglikelihood. Asdiscussed above, the STARTS option of the ANALYSIS command canbe used for automatically generating multiple sets of randomly drawnstarting values that are used to find the best solution.MODEL IDENTIFICATIONNot all models that can be specified in the program are identified. Anon-identified model is one that does not have meaningful estimates forall of its parameters. Standard errors cannot be computed for nonidentifiedmodels because of a singular Fisher information matrix.When a model is not identified, an error message is printed in the output.In most cases, the error message gives the number of the parameter thatcontributes to the non-identification. The parameter to which thenumber applies is found using the TECH1 option of the OUTPUTcommand. Additional restrictions on the parameters of the model areoften needed to make the model identified.417