The terminology introduced by Campbell and Fiske (1959) may be (mis)interpreted that methods are not traits. However, if methods are composites of the causes we want to measure and causes we consider irrelevant, they can be stable dispositions. Both in a substantive sense and on a formal level, no qualitative difference exists between traits and methods. The only difference is that, ideally, traits are single causes (altruistic personality) and methods are composites of causes (altruistic personality + need for approval).
It follows from this view that methods cannot only be "traits" in the formal sense of stable behavioral dispositions; they can also be "states" in the formal sense of systematic individual differences in intraindividual change across time because of the systematic but occasion-specific effects of the measurement situation. In other words, individual differences that stem from shared method variance may not be stable. Self-presentational concerns as a causal component of self-report measures, for instance, may vary systematically across time and situations. At some occasions of measurement, like during a job interview or a date, self-presentational concerns may be stronger than at other occasions. This possibility holds important implications for modeling methods as latent factors in longitudinal multitrait-multimethod designs. It may be appropriate and even necessary in some applications to model latent method factors both as latent states and latent traits. This could be easily done by extending the general latent state-trait framework (Steyer et al., 1992: Steyer, Schmitt, & Eid, 1999) to the domain of methods. Leaving aside issues of model identification, Figure 2.3 depicts the general structure of such a model for two constructs, two occasions of measurement, and two methods with Y k, ST., SS.., LSSRy, MTfe, MSjfc, LMSRjk, and ejjk denoting manifest variables, substantive traits, substantive states, latent substantive state residuals, method trait, method states, latent method state residuals, and measurement error and with i,j, and k denoting the construct, the occasion of measurement, and the method, respectively.
The model shows that methods do not differ in a formal sense from substantive constructs. In fact, this is true for all other facets of the data box. Instead of containing methods, the model in Figure 2.3 could contain modes of behavior. Some modes or types of behavior may be more trait-like than others although both are indicators of the same substantive construct. Therefore, the model in Figure 2.3 could be extended and include latent states and traits for modes of behavior and types of behavior, as well as any other theoretically meaningful facet of the data box.
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