Positive mental wellbeing or symptoms of depression? Discriminant validity of the Warwick-Edinburgh Mental Wellbeing Scale.

Background: Scales for the measurement of mental wellbeing and psychological distress are often used as if they measure different underlying concepts. This assumption is addressed in the present study by examining the discriminant validity of the Warwick-Edinburgh Mental Wellbeing Scale (WEMWBS) with respect to the Patient Health Questionnaire (PHQ-9).

Methods: The present study is based on data (n = 1690) from a baseline data collection which was carried out as part of the evaluation of 'Prompt Mental Health Care' (PMHC), the Norwegian Version of the British 'Improving Access to Psychological Therapies' (IAPT) services. PMHC offers low-threshold treatment for patients with mild to moderate depression or anxiety. Three out of four of the sample were women and three out of four were in the age range 21-50 years. Data were examined by means of structural equation- and latent variable modeling, including bifactor analysis and MIMIC models. Both the 7-item and 14-item versions of the WEMWBS were considered.

Results: (i) The correlations between PHQ-9 and the WEMWBS scales were strong and negative, approaching -0.80 in the latent model analyses with the full (14 items) WEMWBS scale. (ii) Psychometric indices derived from the bifactor models suggested that the WEMWBS-7 and PHQ-9 jointly, and the WEMWBS-14 and PHQ-9 jointly were essentially unidimensional. (iii) The associations between PHQ-9 and a set of demographic variables were similar to associations between the WEMWBS scales and the same set of demographic variables, only with reversed signs. (iv) Associations between the residual WEMWBS scales and a set of demographic variables decreased strongly when removing the reliable variance accounted for by the general depressive symptoms factor.

Conclusion: The results of our study suggest that the WEMWBS may lack discriminant validity with regard to the PHQ-9 in a sample of primary care patients with mild-to-moderate anxiety and/or depression.