Comparing the construct validity of the Patient Health Questionnaire (PHQ-9) and the Major Depression Inventory (MDI) using Rasch analysis

Objective: The Patient Health Questionnaire (PHQ-9) and the Major Depression Inventory (MDI) are psychometric instruments frequently used for depression diagnosis and monitoring in primary care settings. The goal of this study was to assess and compare the construct validity of the PHQ-9 and the MDI using Rasch analysis. Methods: The PHQ-9 and the MDI were distributed in random ordering (first or last) to a waiting room sample of Danish primary care patients. Analysis of fit to the Rasch model, ordering of response categories, dimensionalitytesting, test for differential item functioning, test for local dependency, and calculation of reliability indices were used to evaluate the psychometric characteristics of the instruments. Results: A sample of 281 persons (18-87 years, 72 % females) responded to both instruments. Reliability was acceptable for PHQ-9 (PSI 0.80) and excellent for MDI (PSI 0.88). Both instruments demonstrated fit to the Rasch model after rescoring of all items. Dimensionality testing supports that both instruments are unidimensional. Indication of local dependency was observed for PHQ-9 item 2&6 and MDI item 1&4. No indication of differential item functioning regarding age, gender or ordering of instruments was found. Conclusion: The PHQ-9 and the MDI both demonstrate measurement problems. Modifying individual items and reducing the number of response categories in both instruments could potentially improve their measurement capabilities. Study findings do not provide a conclusive indication of which instrument has superior operating characteristics.