Differential item functioning impact in a modified version of the Roland-Morris Disability Questionnaire

Objective: To evaluate a modified version of the Roland-Morris Disability Questionnaire for differential item functioning (DIF) related to several covariates. Background: DIF occurs in an item when, after controlling for the underlying trait measured by the test, the probability of endorsing the item varies across groups. Methods: Secondary data analysis of two studies of participants with back pain (total n = 875). We used a hybrid item response theory/ logistic regression approach for detecting DIF. We obtained scores that accounted for DIF. We evaluated the impact of DIF on individual and group scores, and compared scores that ignored or accounted for DIF in terms of the strength of association with SF-36 subscale scores. Results: DIF was found in 18/23 items. Salient scale-level differential functioning was found related to age, education, and employment. Overall 24 participants (3%) had salient scale-level differential functioning. Mean scores across demographic groups differed minimally when accounting for DIF. The strength of association of scores with SF-36 scores was similar for scores that ignored and scores that accounted for DIF. Conclusions: The modified version of the Roland-Morris Disability Questionnaire appears to have largely negligible DIF related to the covariates assessed here.