title: Disease-related differential item functioning in the work instability scale for rheumatoid arthritis: converging results from three methods.
abstract: 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.
  
  
  CONCLUSIONS
  RA-WIS scores can be directly compared between RA and OA without significant concerns for DIF-related measurement bias.
authors.affiliation: University of Toronto, Li Ka Shing Knowledge Institute of St. Michael's Hospital, 30 Bond Street, Toronto, Ontario, Canada. ken.tang@utoronto.ca
authors.forename: Kenneth
authors.initials: K
authors.lastname: Tang
mesh_terms: D000328:Adult; D000368:Aged; D001172:Arthritis, Rheumatoid / Q000145:classification / Q000503:physiopathology*; D015331:Cohort Studies; D004185:Disability Evaluation; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D010003:Osteoarthritis / Q000145:classification / Q000503:physiopathology*; D011795:Surveys and Questionnaires / Q000592:standards; D014937:Work; D014938:Work Capacity Evaluation; D055815:Young Adult