Development and Construct Validity of the Work Instability Scale for People With Common Mental Disorders in a Sample of Depressed and Anxious Workers: A Rasch Analysis

BACKGROUND: Sick leave due to common mental disorders, encompassing depression and anxiety disorders, is high. Capturing early signs of reduced function could aid adjustments of work tasks and environment and, thus, endorse a pro-active approach to occupational and health care interventions to prevent long-term sick-leave spells. However, few measurements exist to identify early signs of imbalance, and none that is illness-specific. The aim of this study was to develop a work instability scale for people with common mental disorders and to test the fundamental psychometric properties of the scale. METHODS: Participants were working adults 18-65 years old with depression or anxiety. The scale development started with qualitative interviews (n = 27) which informed the drafting of a dichotomous, self-report questionnaire. Cognitive debriefing (n = 12) was used to check face validity and modify the draft. Internal construct validity of the draft was tested using Rasch analysis (n = 128). The work ability index was used as a comparator measure. RESULTS: The initial 63-item draft showed poor fit to Rasch model expectations. Items displaying poor fit or local response dependency were stepwise removed, resulting in a unidimensional 34-item scale fitting the model expectations, and with no differential item functioning. Person-item threshold distribution showed that the scale is better suited to measure low to moderate work instability, than to measure high instability. Correlations between the newly developed scale and the work ability index showed a significant, moderately strong correlation. CONCLUSIONS: In the initial target sample, the 34-item scale showed acceptable fundamental properties and internal construct validity. Further validation of the scale in a larger sample, including tests for external validity, is warranted.