Construct validity of the DSM-III and RDC classification of melancholia (endogenous depression).

Diagnostic schedules for any psychiatric disorder should represent a coherent concept and demonstrate adequate construct (internal) validity and transferability. Testing fit to latent class models (e.g. by Rasch model fitting test), correspondence to these requirements can be assessed. Based on data from a sample of 173 depressed in-patients, diagnostic schedules for endogenous depression (melancholia) according to DSM-III and RDC are evaluated for their ability to fit the requirements listed above. According to the data presented, the set of DSM-III-criteria for melancholia represents a coherent concept; on the contrary, this is not the case for RDC-criteria. Furthermore, application of the diagnostic algorithm applied to the defining criteria according to DSM-III has been justified; again, this was not the case for the diagnostic algorithm according to RDC. However, a simple linear diagnostic algorithm emerges from our data.