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The purpose of this study was to test the construct validity of a measure of functional ability, developed with the intention of achieving a high degree of variability and capacity for discriminating among a group of healthy elderly people. Data were collected from 734 70 year olds in 1984 and from 395 75 year olds in 1989. Functional ability was measured by traditional activities of daily living. It was developed especially for both healthy and dependent elderly people and included categories in relation to tiredness, reduced speed and dependency of help. Construct validity was tested by the Rasch model for item analysis, addressing specifically the internal validity by assessing the homogeneity of items under different conditions. The analyses showed that the proposed measure of functional ability is formed by 3 scales in relation to tiredness (mobility-T, lower limb-T and upper limb-T) and 2 scales in relation to dependency (mobility-H and PADL-H).
Avlund et al. (1996) studied this question.