A method of interpreting construct validity in terms of the convergent validity and discriminant validity of several different methods of measuring several different psychological attributes or traits. It consists of a matrix (a rectangular table) of correlations between two or more theoretically distinct traits, each measured by two or more different methods. For example, the traits might be extraversion and conscientiousness, assumed on theoretical grounds to be distinct (see Big Five), and each of these traits might be measured by three methods, namely a multiple-choice test, a projective test, and peer ratings; this would produce a matrix with two rows and three columns. The figures along the main diagonal of the matrix, called monotrait-monomethod scores, are estimates of the validity coefficients of the six measures, and for the measurements to be considered satisfactory they should be the highest in the matrix. Correlations between measures of the same trait by different methods, called monotrait-heteromethod scores, provide data about the convergent validity of the measures, on the assumption that if they validly measure the same trait they should correlate with each other, and for the measurements to be considered satisfactory these scores should be reasonably high. Correlations between different traits measured by the same method, called heterotrait-monomethod scores, provide evidence of the degree to which the results are due to a methods factor, irrespective of what is being measured, and if measures of theoretically different traits, using the same method, do not correlate highly with one another, this constitutes evidence of discriminant validity. Finally, correlations between different traits measured by different methods, called heterotrait-heteromethod scores, are generally the lowest in the matrix, and they should not be higher than the validity coefficients in their own rows or columns. The technique was introduced by the US psychologists Donald T(homas) Campbell (1916–96) and Donald W(inslow) Fiske (1916–2003) in an article in the journal Psychological Bulletin in 1959. MTMM abbrev.