ABSTRACT
Erich W. Labouvie asks us to consider to what extent problems arise in longitudinal research because of violations in the principle of equivalence. This chapter provides a convenient framework around which to organize a discussion of empirical longitudinal research. It examines the Labouvie chapter as a reference point to accomplish two tasks: to focus on data from the chapters by Joseph Lowman and David Galinsky and those by B. Comblatt and Yvonne Marcuse to provide a concrete illustration of the analytical difficulties created by one of the problems described by Labouvie, namely the lack of equivalence in measurement. The chapter aims to discuss an additional issue, that of developmental stages, which is crucial to the study of human development within a life-span perspective. In sequential developmental sequences, especially those involving deviant or psychopathological outcomes, the population at each subsequent stage is smaller than the population at the earlier stage.
