A Re-analysis of the Reliability of Psychiatric Diagnosis

Classification systems such as diagnosis have two primary properties, reliability and validity. Reliability refers to the consistency with which subjects are classified; validity, to the utility of the system for its various purposes. In the case of psychiatric diagnosis, the purposes of the classification system are communication about clinical features, aetiology, course of illness and treatment. A necessary constraint on the validity of a system is its reliability. There is no guarantee that a reliable system is valid, but assuredly an unreliable system must be invalid.