the study of a variable or group of variables in the same cases or participants over a period of time, sometimes several years. An example of a longitudinal design is a multiyear comparative study of the same children in an urban and a suburban school to record their cognitive development in depth. A longitudinal study that evaluates a group of randomly chosen individuals is referred to as a panel study, whereas a longitudinal study that evaluates a group of individuals possessing some common characteristic (usually age) is referred to as a cohort study. Also called longitudinal research; longitudinal study. Compare cross-sectional design.