FIGURE 6. Comparison of Cross-Sectional and Structured Multicohort Longitudinal Design Trajectory Estimates^a.

^aThe left panel shows nonlinear, nonparallel trajectories from 100 subjects ages 55–90 years, generated from simulated data. The heavy dark line is a “typical” population trajectory; the heavy red line is the population average of trajectories. The middle panel plots data sampled from trajectories using an accelerated longitudinal design with four yearly measurements. The heavy red line is the data average, and the purple segments are mean within-subject trajectory estimates for evenly spaced age cohorts (baseline ages=55, 57.5, 60, and so on). The right panel plots data from the same accelerated longitudinal design. The heavy red line is again the data average, the heavy black line is a “typical” trajectory, and the heavy green line is the estimated typical trajectory constructed by minimizing age cohort effects.