New directions for the study of within-individual variability in development: the power of "N = 1".

This article provides an introduction to the idiographic approach ("N = 1" research) in developmental psychology and an overview of methodological and statistical techniques employed to address the study of within-individual variability in development. Through a popularization of the idiographic approach and associated statistical techniques, but also through technical advances in the apparatus used to produce single-case intensive longitudinal data, the "power" of "N = 1" is becoming increasingly tangible and may challenge, or supplement, established paradigms in nomothetic (group-level) developmental psychology.