Supplementary Material for: Psychometric properties of the WBIS/-M in a representative prebariatric sample – evidence for an improved 10-item version

Introduction The Weight Bias Internalization Scale and the Modified Weight Bias Internalization Scale are well-established self-report questionnaires for assessing weight bias internalization, which is widespread among bariatric patients. However, among this group, psychometric properties of the Weight Bias Internalization Scale have only been examined in small samples showing unsatisfactory model fit and have not been explored for the modified questionnaire. Methods This study psychometrically evaluated and compared the Weight Bias Internalization Scale and Modified Weight Bias Internalization Scale in a large sample of prebariatric patients (N=825, mean age=46.75 years, SD=11.55) regarding item characteristics, model fit to unidimensionality, reliability, construct validity, and measurement invariance. Results Item 4 of both questionnaires showed low corrected item-total correlations (<.40) and was therefore removed from the scales. The new 10-item versions showed improved item characteristics, internal consistency, model fit to unidimensionality, and convergent and divergent validity when compared to the 11-item versions. The best psychometric properties were found for the 10-item version of the Modified Weight Bias Internalization Scale. Conclusion The 10-item version of the Modified Weight Bias Internalization Scale surpasses the other versions studied in all psychometric properties. Therefore, it should be used in prebariatric patients to detect weight bias internalization and provide them with psychological interventions that could improve bariatric surgery outcomes.