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Disease‐related differential item functioning in the work instability scale for rheumatoid arthritis: Converging results from three methods
Author(s) -
Tang Kenneth
Publication year - 2011
Publication title -
arthritis care and research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.032
H-Index - 163
eISSN - 2151-4658
pISSN - 2151-464X
DOI - 10.1002/acr.20491
Subject(s) - differential item functioning , rasch model , rheumatoid arthritis , logistic regression , item response theory , scale (ratio) , medicine , statistics , physical therapy , clinical psychology , psychometrics , mathematics , physics , quantum mechanics
Objective The 23‐item Work Instability Scale for Rheumatoid Arthritis (RA‐WIS) is a promising measure to assess risk for future work disability. Validated in both rheumatoid arthritis (RA) and osteoarthritis (OA), it has high potential for cross‐disease applications. Our objective was to examine disease‐related differential item functioning (DIF) in the RA‐WIS. Methods Workers with RA (n = 120) or OA (n = 130) were recruited from 3 sites and completed a questionnaire consisting of demographic and health‐ and work‐related variables, including the RA‐WIS (range 0–23, where 23 = highest work instability). Multiple DIF detection methods were applied for comparability: 1) Mantel‐Haenszel and Breslow‐Day procedures, 2) hierarchical 3‐step sequential logistic regression procedure, and 3) a 1‐parameter item response theory approach (Rasch analysis). Both tests of significance (chi‐square and F tests) and effect size statistics (Δ MH , ΔR 2 ) were assessed to confirm items demonstrating uniform or nonuniform DIF. A 2‐step purification procedure was applied to establish a DIF‐free conditioning variable (total RA‐WIS score) for DIF analyses. The resultant impact of disease‐related DIF at the scale level was also evaluated. Results All 3 DIF detection methods converged to reveal 3 RA‐WIS items as having significant disease‐related uniform DIF. Two items (“difficulty opening doors” and “pressure on hand”) were more likely affirmed in RA, while 1 item (“very stiff”) was more likely affirmed in OA. Overall, only a marginal impact at the scale level was found due to a small proportion of scale items exhibiting DIF and the bidirectional nature of DIF effects. Conclusion RA‐WIS scores can be directly compared between RA and OA without significant concerns for DIF‐related measurement bias.