Age-related differences in psychometric properties of World Health Organization Disability Assessment Schedule 2.0 (WHODAS 2.0): a retrospective analysis.

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The aim of this study was to detect a possible age-related differential item functioning (DIF) of the World Health Organization Disability Assessment Schedule 2.0 (WHODAS 2.0) among people with musculoskeletal disorders. This was a cross-sectional study of 1739 consecutive patients with chronic musculoskeletal pain at a university clinic. Difficulty and discrimination parameters were calculated for each of 12 items of the WHODAS 2.0 using item response theory. The presence of DIF was assessed both numerically and graphically. When considering differences in both difficulty and discrimination, a statistically significant DIF was observed for all the items except for the item 'washing' (P values <0.001). The DIF was mostly uniform. Items 'standing', 'household responsibilities', 'learning a new task', 'emotional affectedness', 'concentrating', 'washing' and 'dressing' showed better precision among older patients. Items 'walking', 'dealing with strangers', 'maintaining a friendship' and 'day-to-day work' were more informative among younger patients. Difficulty and discrimination parameters were alike for all 12 items. The DIF was mostly uniform. These results can be taken into account when applying the WHODAS 2.0 to people with musculoskeletal complaints, especially when the studied groups are predominated by people of certain age.