The follow-up: Sample, Variables, and Procedures

Abstract

The 18-to-23-year follow-up study that formed the basis for the chapters in part II is an example of a study that made secondary use of the data files of the original Danish Perinatal Study. The data base used for the part I chapters was subsequently augmented by additional procedures that were designed to optimize the utility of the original data sets for prospectively viewing the development of the cohort over an 18-to-23-year period. Technically, this constituted a follow-up study of individuals at successive stages of life, as contrasted with a typical cohort study covering a specified period of time [Janson 1981]. The distinction between the two types of longitudinal approaches becomes somewhat blurred in the chapters to follow simply because we attempted to reconstruct at the time of follow-up selected elements and events that intervened between the 1959–1961 perinatal inquiry and the 1977–1981 follow-up of the birth cohort. Selection of intervening data items was based on reliability and relevance.