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Objective: The objective of this study was to develop and assess a composite measure of physical capacity using self-report and physical performance items. Method: Item response theory (IRT) is used to evaluate measurement properties of self-report and performance items and to develop a composite measure for 7,609 participants in the National Health and Aging Trends Study. Results: Self-reports distinguish differences at the lower end of physical capacity but not at mid-to-high levels. Performance-based measures discriminate across a fuller spectrum. An IRT-based composite score, drawing on both, provides increased measurement precision across the physical capacity spectrum and detects age group differences if either self-report or performance does so—suggesting it is better suited for studying age-related changes than either measure alone. Discussion: Self-report and performance measures have different strengths on the physical capacity spectrum. IRT provides a means of combining these different measurement approaches for analyses of physical capacity across a broad range of functioning in later life. [ABSTRACT FROM AUTHOR]

Objective: To harmonize measures of cognitive performance using item response theory (IRT) across two international aging studies. Method: Data for persons ≥65 years from the Health and Retirement Study (HRS, N = 9,471) and the English Longitudinal Study of Aging (ELSA, N = 5,444). Cognitive performance measures varied (HRS fielded 25, ELSA 13); 9 were in common. Measurement precision was examined for IRT scores based on (a) common items, (b) common items adjusted for differential item functioning (DIF), and (c) DIF-adjusted all items. Results: Three common items (day of date, immediate word recall, and delayed word recall) demonstrated DIF by survey. Adding survey-specific items improved precision but mainly for HRS respondents at lower cognitive levels. Discussion: IRT offers a feasible strategy for harmonizing cognitive performance measures across other surveys and for other multi-item constructs of interest in studies of aging. Practical implications depend on sample distribution and the difficulty mix of in-common and survey-specific items. [ABSTRACT FROM AUTHOR]