Problems in making inferences from a sample to a population and from one cohort to other cohorts are discussed. It is concluded that in most cases a longitudinal design using repeated measures is preferable to an independent samples design. (Author)

This paper describes the design of the interdisciplinary study of the growth and development of Dutch children in progress at the University of Nymegen, The Netherlands. Schaie's design is based on a trifactorial model which isolates the contributions to developmental data of the factors age, cohort and time of measurement. (Author/MS)