The WHO-5 well-being index – validation based on item response theory and the analysis of measurement invariance across 35 countries

The five-item World Health Organization Well-Being Index (WHO-5) is a frequently used brief standard measure in large-scale cross-cultural clinical studies. Despite its frequent use, some psychometric questions remain that concern the choice of an adequate item response theory (IRT) model, the evaluation of reliability at important cutoff points, and most importantly the assessment of measurement invariance across countries. Data from the 6th European Working Condition survey (2015) were used that collected nationally representative samples of employed and self-employed individuals (N = 43,469) via computer-aided personal interviews across 35 European countries. An in-depth IRT analysis was conducted for each country, testing different IRT assumptions (e.g., unidimensionality), comparing different IRT-models, and calculating reliabilities. Furthermore, measurement invariance analysis was conducted with the recently proposed alignment procedure. The graded response model fitted the data best for all countries. Furthermore, IRT assumptions were mostly fulfilled. The WHO-5 showed overall and at critical points high reliability. Measurement invariance analysis revealed metric invariance but discarded scalar invariance across countries. Analysis of the test characteristic curves of the aligned graded response model indicated low levels of differential test functioning at medium levels of the WHO-5, but differential test functioning increased at more extreme levels. The current study has no external criterion (e.g., structured clinical interviews) to assess sensitivity and specificity of the WHO-5 as a depression screening-tool. The WHO-5 is a psychometrically sound measure. However, large-scale cross-cultural studies should employ a latent variable modeling approach that accounts for non-invariant parameters across countries (e.g., alignment).