Validity of nosological classification

The term "nosological classification" is often used in connection with medical classification systems, and the tendency is to equate it with "diagnosis" and "validity." However, particularly in the case of psychiatry this is far from always being the case. From a scientific point of view, the two most up-to-date classification systems in use today - the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), and the International Classification of Diseases, 10th Revision (ICD-10) - may be considered as the theoretical basis of current psychiatric nosology. In this paper we show that the instrumentally generated DSM-IV or ICD-10 diagnoses of schizophrenia have relatively low validity in comparison with clinician expert diagnoses. If medical classification is to be realistic, simple to use, and reliable, nosological systems must be based not only on established facts, but also on theoretical assumptions regarding the nature of disease.