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14 Error messages 240Alphabetical list of error messages and probable cause(s)/remedieserror messageprobable cause/remedyUnable to invert R or G [US?]matrix:Unable to invert R or G [CORR?]matrix:Variance structure is notpositive definiteXFA model not permitted in RstructuresXFA may not be used as an Rstructurethis message occurs when there is an errorforming the inverse of a variance structure.The probable cause is a non positive definite(initial) variance structure (US, CHOL and ANTEmodels). It may also occur if an identity by unstructured(ID⊗US) error variance model is notspecified in a multivariate analysis (including!ASMV), see Chapter 8. If the failure is on thefirst iteration, the problem is with the startingvalues. If on a subsequent iteration, the updateshave caused the problem. You could tryreducing the updates by using the !STEP qualifier.Otherwise, you could try fitting an alternativeparameterisation. The CORGH modelmay be more stable than the US model.generally refers to a problem setting up themixed model equations. Most commonly, it iscaused by a non positive definite matrix.Use better initial values or a structured variancematricx that is positive definite.You may use FA or FACV. The R structure mustbe positive definite.