The multitrait–multimethod (MTMM) matrix contains the correlations between variables when each variable represents a trait–method unit, that is, the measurement of a trait (e.g., extroversion, neuroticism) by a specific method (e.g., self-report, peer report). In order to obtain the matrix, each trait has to be measured by the same set of methods. This makes it possible to arrange the correlations in such a way that the correlations between different traits measured by the same method can be separated from the correlations between different traits measured by different methods. The MTMM matrix was recommended by Donald T. Campbell and Donald W. Fiske as a means of measuring the convergent and discriminant validity. This entry discusses the structure, evaluation, and analysis approaches of the MTMM matrix.