Idiographic assessment: conceptual and psychometric foundations of individualized behavioral assessment.

Idiographic assessment is the measurement of variables and functional relations that have been individually selected, or derived from assessment stimuli or contexts that have been individually tailored, to maximize their relevance for the particular individual. This article first reviews various definitions and clinical applications of idiographic assessment. Several properties of behavior problems and causal relations provide the conceptual basis for idiographic assessment: (a) differences across persons in the attributes, response systems, and dimensions of a behavior problem, and (b) differences across persons in the causal relations relevant to a particular behavior problem. Because of these individual differences, nomothetic measures often reflect variance that is irrelevant to the targeted construct for the individual. We present a psychometric framework for idiographic assessment by first summarizing why the psychometric principles used to develop standardized measures of nomothetic constructs can create incongruity between the nomothetic measure and the characteristics of the targeted construct for an individual. We then develop a psychometric framework for idiographic assessment that combines components of multilevel modeling (random effects) and confirmatory factor analyses applied to repeated measurements of each individual. We also provide a step-by-step guide for the development and evaluation of an idiographic assessment instrument.