ABSTRACT
The study of variability has long been a central focus in psychology. In the late 1950s, Cronbach (1957) made the distinction between two disciplines in scientific psychology: experimental and correlational psychology. Although the focus of the former is on variability induced through manipulation under experimenter control, the focus of the latter is on variation that exists naturally, such as individual differences. Although each approach to the study of variability has its merits, the correlational approach may be the only option for the scientific study of constructs that an experimenter is not able to manipulate (e.g., IQ). As the correlational approach has and continues to be used extensively for studying variability attributable to individual differences, there has been incredible growth and diversification of the study of variability in behavioral and psychological research, within persons over time, or intraindividual variability (Cattell, 1966; Fiske & Rice, 1955; Nesselroade & Featherman, 1997). In particular, intraindividual variability has emerged as an important phenomenon for studying dynamic psychological constructs and processes, especially in adulthood and old age (Nesselroade & Ram, 2004; Ram & Gerstorf, 2009).
