Although the implications of multidetermination were discussed with regard to methods, they apply to all facets of a data box. The degree of relative consistency across time, situations, and other facets always depends on the relative weights of common and unique factors. Again, this has important consequences for psychological measurement (e.g., if helping in different situations depends only on altruistic personality, individual differences in helping remain nonspecific across situations). Every single act is then a perfect measure of altruistic personality. In contrast, if helping was caused by different specific factors in different situations with altruistic personality being the only common source of individual differences, the generalizability of individual differences across situations is then limited. Accordingly, the construct validity of each act as a measure of altruistic personality is also limited.
The reasoning also applies when differences on facets other than the person facet are of interest (Shadish et al., 2002; Wittmann, 1988). In general psychology, we want to discriminate among situations and replicate situation differences across other facets. In educational psychology and intervention research, we want to discriminate among time points and obtain consistent changes across other facets. Sometimes we are even interested in generalized differences between methods. We might want to know, for instance, whether better grades are given in oral versus written exams or whether grades differ systematically among teachers. Although method differences of this kind are undesirable in many research contexts because they limit the comparability of results, exploring systematic differences between methods can be important for making them comparable (Hoyt, 2000).
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