model. @grayson_identification_1994 showed that this is correct, for the CTM and CTCM models, which introduces correlation parameters among methods (CTCM) and, additionally, between methods and traits (CTM). Second, @grayson_identification_1994 [p. 130] also suggested conditions under which the CTUM under consideration here would be identified. These conditions are met under conditions 1 and 2 above. However, these authors did not consider the *reduced-group* split-ballot model; in this, present, model, the missing heteromethod moments generate the rank deficiency above. This fact explains observations by @revilla_split-ballot_2013 that the observed nonconvergence and stability issues with reduced-group SBMTMM disappear when a third group including these heteromethod moments is included in the analysis. Finally, @saris_new_2004 remarked that rank-deficiency of the reduced-group SBMTMM model occurs whenever condition 2 above is fulfilled. Our analysis indicates that this is not sufficient to generate a rank deficiency, but both conditions 1 and 2 must be fulfilled. At the same time, as noted by @revilla_split-ballot_2013, problems are indeed observed primarily when condition 2 is approached. The following sections bear out this observation.

Experiments
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Shapiro’s example: rank deficiency versus identification
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To illustrate the problem of rank deficiencies in a simpler case than reduced-group split-ballot multitrait-multimethod models, we will first discuss 