Abnormal psychology: chapter 3 classification and diagnosis. Diagnosis: consists of a determination or identification of the nature of a person"s disease or condition or a statement of that finding. Why do we need a classification system for mental disorders. Classification/the ability to categorize information allows scientists to better identify and understand various phenomena. Yields information that serves as the basis for a diagnosis. The perfect diagnostic system would classify disorders on the basis of a study of presenting symptoms (pattern of behaviours), etiology (history of development), prognosis (future of development) and response to treatment. A good diagnostic system is very useful , if not essential - what makes the system useful or not: reliability: must give the same measurement for a given thing every time. A type of reliability that is relevant when evaluating a diagnostic system is inter-rater reliability: which refers to the extent of which two clinicians agree on the diagnosis of a particular patient.