This chapter focuses on several aspects of time and the way in which processes of within-person change can be modeled using repeated measures in longitudinal designs. When events occurring in historical time affect only distinctive (or unique) subpopulations, concepts other than period effects are used. One such type of effect refers to the potential of historical events to affect only one segment of the age distribution. The dimension of time concerned with the lives of individuals, usually conceptualized in terms of the biological, psychological, and social processes that shape the life cycle and aging of individuals, is referred to as biographical time. A number of different conceptions of time are useful as a way of thinking about individual development and change. The most common are age and life cycle-one continuous and one discrete-both of which measure time from birth. The chapter focuses on five central issues that are uniquely suited to longitudinal analysis using growth curve models: specifying individual trajectories of within-person change, modeling individual differences in trajectories of within-person change, specifying the determinants and/or predictors of individual differences in patterns of change, testing for cohort differences in patterns of within-person change, and modeling the effects of events and transitions on processes of change.
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