Critical issues regarding the utility of age-heterogeneous cross-sectional samples for estimating associations among rates of change are briefly reviewed and further examined in response to the four insightful commentaries on our original discussion paper. These general issues are considered in terms of current practice in gerontological studies. Several of the commentaries refer to additional problems (e.g., selection/attrition, cohort differences) with cross-sectional designs for inferring correlated rates of change, often modeled as shared age-related variance. The commentators concurred with, and performed simulations that support, our primary contention that the influence of mean population trends over time has a major confounding influence on cross-sectional estimates of association. While this confound has been acknowledged and demonstrated in a number of publications over this century, cross-sectional studies and covariance modeling remain common practice in gerontological studies. We question the value of this practice for advancing aging theory and would like to see greater emphasis placed on longitudinal studies and the use of cross-sectional designs and analytic methods that permit separation of age-related mean trends from estimates of association between age-related variables.

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