Many psychologists collect multitrait-multimethod (MTMM) data to assess the convergent and discriminant validity of psychological
measures. In order to choose the most appropriate model, the types of methods applied have to be considered. It is shown how the combination of
interchangeable and structurally different raters can be analyzed with an extension of the correlated trait-correlated method minus one
[CTC(M-1)] model. This extension allows for disentangling individual rater biases (unique method effects) from shared rater biases (common
method effects). The basic ideas of this model are presented and illustrated by an empirical example.