In a review of the past and current literature on the classification of depression as a psychiatric disorder, it is apparent that diagnosis and treatment go hand-in-hand. The original Kraepelinian concept of dividing depression by etiological factors into endogenous and exogenous has persisted so that modern thinking in psychiatry still espouses aspects of this in various guises.
While the art of diagnosis and treatment may have declined, the science of diagnosis and treatment has made great strides with the advent of high-speed computers, sophisticated statistical analyses, and the birth of neuropsychopharmacology as a science.
I will summarize my own work on the formulation of an operational definition of depression as a disorder, its application into a rating instrument, the various validity and reliability studies performed, as well as clinical application of the rating scale.
Zung WWK. From Art to ScienceThe Diagnosis and Treatment of Depression. Arch Gen Psychiatry. 1973;29(3):328-337. doi:10.1001/archpsyc.1973.04200030026004