Comparing alternative measures of functional limitation

Background: Functional limitation is a central concept in the disability process, but its operationalization has varied widely, making it impossible to compare results across studies. Objective: Our goal is to systematically compare the effectiveness of alternative measures of functional limitations to predict disability and to provide guidelines for their standardization. Design: Over 100 alternative scales of functional limitations are compared in regression models of disability. The Bayesian information criterion is used to compare the performance of measures. Subjects: The subjects were 5764 women, aged 37 to 68, from the Mature and Young Women's cohorts of the National Longitudinal Surveys. Measures: Scales are constructed from 9 indicators of activity limitations: (1) grasping, (2) lifting/carrying up to 10 pounds, (3) lifting/carrying heavy weights, (4) reaching, (5) sitting, (6) stairs, (7) standing, (8) stooping, and (9) walking. Respondents were asked whether they could perform the activity without difficulty, with difficulty, or could not perform it at all. The 4 disability outcomes include binary and ordinal measures of work limitation, a single ADL measure (bathing), and total number of ADLs. Results: The 3 best scales used all 9 indicators. The top scale truncated values above 5, while the others logged the sum. Two scales were based on sums of binary indicators, while the third used 3 category indicators. Conclusions: Simple scales perform better than complex scales that separate upper and lower body limitations or weight some limitations more than others. Scales that transform higher values are preferable.