Introduction and Section Overview

Stationarity is a mathematical property that allows a dynamic system to repeat itself indefinitely according to some prescribed laws. Most models of intraindividual change-including longitudinal panel models, time series models, dynamical systems models, and state-space models-assume some level of stationarity. Strict stationarity refers to the invariance of all distributional characteristics of a set of data over time. Weak stationarity, namely invariance of the mean and covariance structures of a data set, is routinely assumed in many modeling contexts.