Rasch analysis informed modifications to the Work Instability Scale for Rheumatoid Arthritis for use in work-related upper limb disorders

OBJECTIVE: The Work Instability Scale for Rheumatoid Arthritis (RA-WIS) is a promising prognostic tool for future work disability outcomes. Rasch analysis was conducted to examine the psychometric performance of the RA-WIS in work-related upper limb disorders. STUDY DESIGN AND SETTING: Eligible injured workers (n=396) attending a Shoulder and Elbow Specialty clinic participated in a 1-year study with surveys fielded at four time points. Fit of RA-WIS data (range, 0-23 with 23=highest work instability) to the Guttman structure was evaluated by item-fit, person-fit, item-trait interaction statistics, and the person separation index (PSI). Differential item functioning (DIF) was evaluated by two-way analyses of variance of the residuals across age, sex, location of injury, perceived exertion at work, and repeated testing over time. Unidimensionality was evaluated by principal component analysis of residuals and tests of local independence. RESULTS: RA-WIS data showed significant deviations from the Guttman structure (item-trait interaction chi(2)=181.6, P<0.0001, PSI=0.86). A sequential removal of the six most misfitting items was performed, resulting in a 17-item scale that met all Rasch model expectations (chi(2)=57.5, P=0.007, PSI=0.83), including unidimensionality, local independence of items, and the absence of DIF across all tested factors. CONCLUSION: A new 17-item Upper Limb Work Instability Scale that satisfied assumptions for interval-level scaling was derived