Psychometric testing on two weight stigma instruments in Iran: Weight Self-Stigma Questionnaire and Weight Bias Internalized Scale

Purpose: This study aimed to comprehensively and thoroughly examine the psychometric properties of two commonly used weight-related self-stigma instruments on Iranian adolescents with overweight or obesity: Weight Self-Stigma Questionnaire [WSSQ] and Weight Bias Internalization Scale [WBIS]. Methods: After ensuring the linguistic validity of both the WSSQ and WBIS in their Persian versions, 737 Iranian adolescents with overweight or obesity (male = 354; mean age = 15.8 ± 1.3 years; body mass index = 30.0 ± 4.8 kg/m²) completed both questionnaires and other relevant measures regarding their depression, anxiety, stress, dietary self-efficacy, weight efficacy lifestyle, quality of life, body fat, self-esteem, body shape preoccupation, and sleepiness. Results: In the scale level, the confirmatory factory analysis verified the two-factor structure for the WSSQ and the single-factor structure for the WBIS. The factorial structures were further found to be invariant across gender (male vs. female) and across weight status (overweight vs. obesity). Additionally, both the WSSQ and WBIS had promising properties in internal consistency, test–retest reliability, separation reliability, and separation index. In the item level, all items but WBIS item 1 (infit mean square = 1.68; outfit mean square = 1.60) had satisfactory properties in factor loadings, corrected item-total correlation, test–retest reliability, and infit and outfit mean square. Moreover, all the items did not display substantial differential item functioning (DIF) across gender and across weight status. Conclusion: Both the WSSQ and WBIS were valid instruments to assess the internalization of weight bias for Iranian adolescents with overweight or obesity. Level of evidence: Level V, cross-sectional descriptive study.