Methodological challenges in measurements of functional ability in gerontological research. A review.

This article addresses two important challenges in the measurement of functional ability in gerontological research: the first challenge is to connect measurements to a theoretical frame of reference which enhances our understanding and interpretation of the collected data; the second relates to validity in all stages of the research from operationalization to meaningful follow-up measurements in longitudinal studies. Advantages and disadvantages in different methods to do the measurements of functional ability are described with main focus on frame of reference, operationalization, practical procedure, validity, discriminatory power, and responsiveness. In measures of functional ability it is recommended: 1) always to consider the theoretical frame of reference as part of the validation process (e.g., the theory of "The Disablement Process"; 2) always to assess whether the included activities and categories are meaningful to all people in the study population before they are combined into an index and before tests for construct validity; 3) not to combine mobility, PADL and IADL in the same index/scale; 4) not to use IADL as a health-related functional ability measure or, if used, to ask whether problems with IADL or non-performance of IADL are caused by health-related factors; 5) always to make analyses of functional ability for men and women separately as patterns of functional ability and patterns of associations between other variables and functional ability often vary for men and women; and 6) to exclude the dead in analyses of change in functional ability if the focus is on predictors of deterioration in functional ability.