Factor structure of sizing me up, a self-reported weight-related quality of life instrument, in community children across weight status

Background: To further understand the psychometric properties of the Sizing Me Up - a newly developed quality of life (QoL) instrument related to weight status. We extended the target population to a community sample, including obese, overweight, normal-weight, and underweight children. Methods: Based on the data of 497 students in third to sixth grades, we used the following approaches: analysis of variance to detect the QoL scores among groups; confirmatory factor analysis (CFA) to examine the original structure; and Rasch analysis to test the misfit items, disordered response descriptors, and differential item functioning (DIF) items. Results: Obese children had the lowest QoL total score; overweight children had lower QoL total score than normal-weight children; and underweight children had lower QoL in social avoidance than normal-weight children (all p < 0.01). CFA suggested that the factor structure is acceptable. Rasch corroborated that there were no misfit items; however, 10 items showed disordered thresholds in response descriptors and 6 items displayed significant DIF. Conclusions: Sizing Me Up can be applied to underweight children in the community and it can yield valid and reliable scores. Future studies are needed to explore the issues of disordered response descriptors and DIF items. Additional revisions for the Sizing Me Up may be warranted.