Purpose: Developing valid and reliable instruments that can be used across countries is necessary. The present study aimed to test the comparability of quality of life scores across three European countries (Finland, Poland, and Spain).
Method: Data from 9987 participants interviewed between 2011 and 2012 were employed, using nationally representative samples from the Collaborative Research on Ageing in Europe project. The WHOQOL-AGE questionnaire is a 13-item test and was employed to assess the quality of life in the three considered countries. First of all, two models (a bifactor model and a two-correlated factor model) were proposed and tested in each country by means of confirmatory factor models. Second, measurement invariance across the three countries was tested using multi-group confirmatory factor analysis for that model which showed the best fit. Finally, differences in latent mean scores across countries were analyzed.
Results: The results indicated that the bifactor model showed more satisfactory goodness-of-fit indices than the two-correlated factor model and that the WHOQOL-AGE questionnaire is a partially scalar invariant instrument (only two items do not meet scalar invariance). Quality of life scores were higher in Finland (considered as the reference category: mean = 0, SD = 1) than in Spain (mean = - 0.547, SD = 1.22) and Poland (mean = - 0.927, SD = 1.26).
Conclusions: Respondents from Finland, Poland, and Spain attribute the same meaning to the latent construct studied, and differences across countries can be due to actual differences in quality of life. According to the results, the comparability across the different considered samples is supported and the WHOQOL-AGE showed an adequate validity in terms of cross-country validation. Caution should be exercised with the two items which did not meet scalar invariance, as potential indicator of differential item functioning.
Keywords: Bifactor model; Measurement invariance; Multi-group confirmatory factor analysis; Quality of life; WHOQOL-AGE.