Deriving an overall appearance domain score by applying bifactor IRT analysis to the BODY-Q appearance scales

Abstract

Purpose With the BODY-Q, one can assess outcomes, such as satisfaction with appearance, in weight loss and body contouring patients using multiple scales. All scales can be used independently in any given combination or order. Currently, the BODY-Q cannot provide overall appearance scores across scales that measure a similar super-ordinate construct (i.e., overall appearance), which could improve the scales' usefulness as a benchmarking tool and improve the comprehensibility of patient feedback. We explored the possibility of establishing overall appearance scores, by applying a bifactor model to the BODY-Q appearance scales. Methods In a bifactor model, questionnaire items load onto both a primary specific factors and a general factor, such as satisfaction with appearance. The international BODY-Q validation patient sample (n = 734) was used to fit a bifactor model to the appearance domain. Factor loadings, fit indices, and correlation between bifactor appearance domain and satisfaction with body scale were assessed. Results All items loaded on the general factor of their corresponding domain. In the appearance domain, all items demonstrated adequate item fit to the model. All scales had satisfactory fit to the bifactor model (RMSEA 0.045, CFI 0.969, and TLI 0.964). The correlation between the appearance domain summary scores and satisfaction with body scale scores was found to be 0.77. Discussion We successfully applied a bifactor model to BODY-Q data with good item and model fit indices. With this method, we were able to produce reliable overall appearance scores which may improve the interpretability of the BODY-Q while increasing flexibility.