ABSTRACT
Statistical methodology developed in psychology is mostly applied to a collection of individuals rather than to a single person (Kratochwill, 1978, p. 3). The development of psychological testing methods in the first half of the 20th-century put the individual on the background as the initial objective was to differentiate among individuals. Keeping this in mind, the deliberate focus on advancement of analysis techniques based on variation between individuals (inter-individual variation, IEV) instead of variation within a single individual seems tenable. However, models for time-dependent variation of a single individual (intra-individual variation, IAV) have been widely available for some time. The discovery of the intrinsically stochastic timedependent behavior within grains of pollen (Brownian motion) led to the development of appropriate models for single systems in the beginning of the 20th century. In this regard, the absence of a pure N = 1 perspective in psychometrics might be perceived as startling.
