Finding salvage in standardization? Structural and longitudinal evaluation of the patient health questionnaire anxiety and depression scale in a UK representative sample

Abstract Objectives Some research funders mandate the use of the Patient Health Questionnaire‐9 (PHQ‐9) and Generalized Anxiety Disorder‐7 (GAD‐7) as independent measures of depression and anxiety. This study examined their independence in a UK representative sample by testing the structural validity, external validity, longitudinal measurement invariance and growth trajectories of the Patient Health Questionnaire Anxiety and Depression Scale (PHQ‐ADS). Methods Data from Waves 1 ( N = 2025), 2 ( N = 1406) and 3 ( N = 1166) of the COVID‐19 Psychological Research Consortium Study were analysed. Confirmatory factor analysis and exploratory structural equation modelling were used to compare PHQ‐ADS models. Structural equation modelling examined associations with Cybernetic Big Five Theory traits, intolerance of uncertainty and mental health treatment status. Longitudinal measurement invariance was tested across waves, and curve‐of‐factors models examined change over time. Results A bifactor model with a correlated residual produced interpretable factors. Auxiliary bifactor statistics supported a composite PHQ‐ADS score, whereas PHQ‐9 and GAD‐7 summed scores showed little reliable unique variance after accounting for the general factor. Stability and Neuroticism were associated with the general factor ( p < .001), Conscientiousness with the PHQ‐9 group‐factor ( p < .001) and Agreeableness with the GAD‐7 group‐factor ( p = .003). Intolerance of uncertainty was associated with the general and GAD‐7 group‐factors ( p < .001). Mental health treatment status was primarily associated with the general factor. Residual invariance was supported after inclusion of a trait‐factor, and growth trajectories differed across factors. Conclusions Although the PHQ‐9 and GAD‐7 showed some tentative uniqueness within latent models, their summed scores would largely reflect general distress. These findings suggest caution when interpreting PHQ‐9 and GAD‐7 total scores as distinct indicators, warranting reconsideration of measurement standardization.