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Appendix
D
Numeric Diagnosis of
Non-Identifiability
In order to decide whether a parameter is identified or an entire model is identified,
Amos examines the rank of the matrix of approximate second derivatives and of some
related matrices. The method used is similar to that of McDonald and Krane (1977).
There are objections to this approach in principle (Bentler and Weeks, 1980;
McDonald, 1982). There are also practical problems in determining the rank of a
matrix in borderline cases. Because of these difficulties, you should judge the
identifiability of a model on a priori grounds if you can. With complex models, this
may be impossible, so you will have to rely on the numeric determination of Amos.
Fortunately, Amos is pretty good at assessing identifiability in practice.