The weight-specific adolescent instrument for economic evaluation (WAItE): psychometric evaluation using a Rasch model approach

Purpose: The Weight-specific Adolescent Instrument for Economic evaluation (WAItE) is a 7-item condition-specific tool assessing the impact of weight status on seven dimensions of quality of life. The content of the WAItE was developed with both treatment-seeking and non-treatment-seeking adolescents aged 11–18 years. The aim of this study was to assess the psychometric properties of the WAItE in adolescent and adult populations.

Methods: Treatment-seeking adolescents with obesity (females n = 155; males n = 123; mean age = 13.3; 13.1 years, respectively) completed the WAItE twice. An adult general population sample completed the WAItE via an online survey (females n = 236; males n = 231; mean age = 41.2; 44.3 years, respectively). The Partial Credit Model was applied to the data and item fit evaluated against published criteria.

Results: The WAItE had a unidimensional structure both for adolescents and adults. There was no item misfit observed for either participant samples and no differential item functioning (DIF) was present by age or gender for the adolescents. Some DIF was observed across age groups for the adult sample. For the adolescent sample, stable item locations were observed over time.

Conclusions: The aim of the WAItE is to assess the impact of weight status on the lives of adolescents in cost-effectiveness evaluation of weight management programmes. The results of this study demonstrated that the WAItE has reliable psychometric properties. The instrument may therefore be used to aid informed decision around the identification of cost-effective weight management programmes in both adolescent and adult populations.