Relative consistency, relative stability, generalizabil-ity, reliability, convergence, and convergent validity are closely related concepts, introduced separately because they have been used in the literature for denoting different substantive applications and interpretations of the same general principle—the principle of covariation. The concepts of specificity, interaction, divergence, and discriminant validity are also closely related to each other and commonly denote a lack of covariation.
The relation among the concepts of consistency and specificity becomes evident from the use of these concepts in statistical analyses of the data box. All concepts can be and have been defined mathematically, and these definitions are either identical or closely related. Most mathematical definitions of consistency and specificity stem from two well-known statistical coefficients: the coefficient of variance and the coefficient of covariance. This is true for the Pearson correlation coefficient, the multiple correlation coefficient, coefficients of determination, intraclass correlation coefficients, as well as for other coefficients of relative consistency and convergence proposed in generalizability theory (Cron-bach, Gleser, Nanda, & Rajaratnam, 1972) and multivariate reliability theory (Wittmann, 1988).
The general linear model is another mathematical construct that unifies on a formal level many of the substantive principles discussed in this section. The general linear model makes the principle of multidetermination concrete in the language of algebra. It serves as a common formal denominator of many statistical procedures developed for the analysis of consistency and specificity (e.g., analysis of variance, factor analysis, and the more recent and more sophisticated methods of modeling covariance structures among facets of the data box (Eid, 2000; Jôreskog, 1969; Kenny & Kashy, 1992; Kenny & Zautra, 2001; Marsh, 1989; Steyer et al., 1992, Steyer, Schmitt, & Eid, 1999; Widaman, 1985). Several chapters of this volume will provide a detailed analysis of the formal and mathematical commonalities among the concepts of consistency, specificity, and multidetermination introduced here on a conceptual level.
LOOKING BACK: HISTORICAL MILESTONES IN MULTIMETHOD ASSESSMENT
The history of multimethod thinking in psychological assessment and construct validation can be described metaphorically as an avenue from an (unknown) starting point to an (unknown) end point from which many roads stem. Some of them turn back to the main route, whereas others become dead end streets. These pathways cannot and do not need a detailed description here. Rather, I concentrate on important milestones along the developmental route of the general multimethod approach. As I see it, the most important milestones are Brunswik's (1934) work on probabilistic func-tionalism in human perception, Campbell and Fiske's (1959) multitrait-multimethod (MTMM) matrix, covariance structure modeling based on Joreskog's (1969) confirmatory factor analysis (CFA), Generalizability Theory (Cronbach et al., 1972), and Critical Multiplism (Cook, 1985).
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