Purpose: The aim of this study was to examine fit of the original 24-item Roland-Morris Disability Questionnaire and three short-form versions to a Rasch model with particular attention to targeting of item difficulty and to differential item functioning.
Methods: Cross-sectional survey of 140 people with low back pain seeking physiotherapy treatment. Data were analysed using a dichotomous Rasch model.
Results: All versions showed adequate overall data fit to the Rasch model, with few misfitting items. Person separation was around 0.85 for all versions. Item 5 (use a handrail to get upstairs) showed differential item functioning by age. Targeting of persons of high ability was poor and short-form versions also had poor targeting of persons of low ability. Items of similar difficulty clustered in the centre of the logit scale.
Conclusion: Although the Roland data fit the Rasch model, there were insufficient items of higher difficulty to sufficiently evaluate disability in persons with mild disability. Short-form versions also lacked items of lower difficulty.