[Methodological challenges in measurements of functional ability in gerontological research]

This article addresses the advantages and disadvantages of different methods in measuring functional ability with its main focus on frame of reference, operationalization, practical procedure, validity, discriminatory power, and responsiveness. When measuring functional ability it is recommended: 1) Always to consider the theoretical frame of reference as part of the validation process. 2) Always to assess the content validity of items before they are combined into an index and before performing 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 analyse functional ability separately for men and women. 6) To exclude the dead in analyses of change in functional ability if the focus is on predictors of deterioration in functional ability.