Differential item functioning in a computerized adaptive test of functional status for people with shoulder impairments is negligible across pain intensity, gender, and age groups

People with shoulder impairments (N = 3,767) reported upper extremity function using a 37-item shoulder-specific computerized adaptive test (shoulder CAT). The authors determined whether items of the shoulder CAT have differential item functioning (DIF) by pain intensity (low and high), gender (men and women), and age groups (young-adult, middle-aged and oldadult). They assessed whether items have uniform and/or non-uniform DIF using an ordinal logistic regression and item response theory approaches and applied large and small DIF criteria to assess the magnitude of DIF. The analyses revealed that uniform DIF was absent in all 37 items. Only six items exhibited non-uniform DIF using the large DIF criterion. Adjusting the person-ability measures for DIF had minimal practical impact on the overall measure of shoulder function estimated using the shoulder CAT. The shoulder CAT provided a precise measurement of function without discriminating for pain intensity, gender, and age among patients referred to rehabilitation with shoulder impairment.