Evaluation of the Structural Validity of the Work Instability Scale Using the Rasch Model

OBJECTIVE: To use Rasch analysis to examine the measurement properties of the 23-item version of the Work Instability Scale (WIS-23) in a sample of worker compensation claimants with upper extremity disorders. DESIGN: Secondary data analysis on the data retrieved from a cross-sectional study. SETTING: Tertiary care hospital. PARTICIPANTS: Patients (N=392) attending a specialty clinic for workers with upper limb injuries at a tertiary hospital were prospectively enrolled. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: WIS-23. RESULTS: The study sample contained 392 participants between the ages of 19 and 73 years (mean, 47.0±10.5y). There were 148 (37.8%) women, 182 (46.4%) men, and 62 (15.8%) participants for whom sex identification was unavailable. The initial WIS data analysis showed significant misfit from the Rasch model (item-trait interaction: χ²=293.52; P<.0001). Item removal and splitting were performed to improve the model fit, resulting in a 20-item scale that met all assumptions (χ²=160.42; P=.008), including unidimensionality, local independence of items, and the absence of differential item function based on age, sex of respondents, employment type, and affected upper extremity area across all tested factors. CONCLUSION: With the application of Rasch analysis, we refined the WIS-23 to produce a 20-item WIS for work-related upper extremity disorders (WIS-WREUD). The 20-item WIS-WREUD demonstrated excellent item and person fit, unidimensionality, acceptable person separation index, and local independency. The WIS-20 may provide better measurement properties, although longitudinal psychometric evaluations are needed.