Validity is about the meaning of test scores (Cronbach & Meehl, 1955). Although a variety of narrower definitions have been proposed, psychometric validity deals with the extent to which test scores exclusively measure their intended psychological construct(s) and guide consequential decision-making. This concept represents something of a metamorphosis in understanding test validation because of its emphasis on the meaning and application of test results (Geisinger, 1992). Validity involves the inferences made from test scores and is not inherent to the test itself (Cronbach, 1971).
Evidence of test score validity may take different forms, many of which are detailed below, but they are all ultimately concerned with construct validity (Guion, 1977; Messick, 1995a, 1995b). Construct validity involves appraisal of a body of evidence determining the degree to which test score inferences are accurate, adequate, and appropriate indicators of the examinee's standing on the trait or characteristic measured by the test. Excessive narrowness or broadness in the definition and measurement of the targeted construct can threaten construct validity. The problem of excessive narrowness, or construct underrepresentation, refers to the extent to which test scores fail to tap important facets of the construct being measured. The problem of excessive broadness, or construct irrelevance, refers to the extent to which test scores are influenced by unintended factors, including irrelevant constructs and test procedural biases.
Construct validity can be supported with two broad classes of evidence: internal and external validation, which parallel the classes of threats to validity of research designs (D. T. Campbell & Stanley, 1963; Cook & Campbell, 1979). Internal evidence for validity includes information intrinsic to the measure itself, including content, substantive, and structural validation. External evidence for test score validity may be drawn from research involving independent, criterion-related data. External evidence includes convergent, discriminant, criterion-related, and consequential validation. This internal-external dichotomy with its constituent elements represents a distillation of concepts described by Anastasi and Urbina (1997), Jackson (1971), Loevinger (1957), Messick (1995a, 1995b), and Millon et al. (1997), among others.
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