ABSTRACT. Background: Disability questions require older people to report difficulties with everyday activities, using broad categorical responses. Relatively little is known about population group differences in the thresholds for reporting difficulty or inability with medium-distance mobility against tested mobility-related performance. We aimed to estimate the thresholds on tested performance at which self-reports change from one category to another, across a range of sociodemographic subgroups. We also aimed to compare reported and tested performance across two national population studies. Methods: The samples were from the third U.S. National Health and Nutrition Examination Study (NHANES III) and the Longitudinal Aging Study Amsterdam (LASA). Measures of gait speed, chair stands, and peak expiratory flow rate in both studies yielded the validated index of mobility-related physical limitations (MOBLI). Latent probit models were used to estimate cutpoints (thresholds) on the index for reporting difficulty or inability to walk a medium distance. Results: Thresholds for reporting difficulty or inability were studied by age, sex, race, educational level, and income in NHANES III. In models adjusting for the other factors, performance thresholds for reporting disability categories varied by age and income. The younger elderly persons in NHANES III on average reported difficulties or inabilities only when they reached a more severe level of tested limitation compared with older old persons. A similar pattern exists for those on higher incomes. For race, differences in threshold were present only for reporting inability, but not difficulty. Significant differences in thresholds were not present between groups defined by sex or for years of education. Comparisons between the NHANES and LASA studies show that lower reported mobility difficulty or inability prevalence in the Dutch sample is attributable both to reporting at higher levels of limitation and to better functioning. Conclusions: There is evidence of differences in thresholds for reporting mobility disability, especially across age and income groups in older Americans. Further work is needed to understand the perceptual, attitudinal, or environmental factors that cause these reporting differences.
