Purpose
The Major Depression Inventory (MDI) is a widely used self-rating depression scale commonly in primary care in Denmark. It has not been subject to robust psychometric validation in a general population setting. The aim of this study was to evaluate the psychometric measurement properties of the MDI when applied in the general population.

Methods
We evaluated statistical psychometric validity using modern test theory (confirmatory factor analysis, item response theory models and Rasch measurement theory) testing local independence and differential item function across groups defined by gender, age, education, and chronic disease status. Separate analyses across different strata and across different statistical models were employed.

Results
Regarding structural validity we consistently identified local dependence for the item two pairs (MDI2,MDI3) and (MDI4,MDI5) across strata. This result was confirmed by bifactor CFA models and item screening. We further identified substantial differential item functioning with respect to age group and with respect to chronic disease. We identified quantified the magnitude of this lack of measurement invariance.

Conclusion
The MDI is psychometrically valid in homogenous sub populations, but the disclosed evidence of local dependence means that published estimates of its reliability cannot be trusted. The lack of measurement invariance means that the instrument cannot be used to compare individuals or groups unless they are similar in terms of age group and chronic disease status.