Evaluating Convergence of Within-Person Change and Between-Person Age Differences in Age-Heterogeneous Longitudinal Studies.

The distinction of between-person age differences from within-person age changes is necessary for understanding aging-related change processes. Although longitudinal studies are required to address issues relating to within person change, most studies begin using age-heterogeneous samples and conclude using survival-heterogeneous samples. Given the numerous potential confounds associated with age-heterogeneous samples, careful treatment of between-person age differences is essential to obtain the correct inferences regarding within-person age change. We demonstrate how failure to differentiate between-person age effects (and by extension, of survival age or other effects producing sample heterogeneity) will lead to uninterpretable inferences regarding within-person change. We recommend that convergence of age differences and age changes be formally evaluated whenever possible.