Epidemiology is concerned with understanding and controlling disease epidemics by investigating empirically the associations between variation in exposure to disease-causing agents external to the individual, variation in the resistance of individuals exposed to the disease-causing agents, and variation in resistance resources in the environments of exposed individuals . These investigations are initially carried out by examining natural variations. Hypotheses based on these analyses are then, usually, tested provisionally in naturalistic quasi-experimental situations with matching or statistical controls used to approximate the conditions of an experiment. If the hypotheses stand up to these preliminary tests, they are evaluated in interventions aimed at preventing the onset or altering the course of the disorders. Psychiatric epidemiology traditionally lags behind other branches of epidemiology because of difficulties encountered in conceptualizing and measuring mental disorders. As a result, much contemporary psychiatric epidemiology continues to be descriptive, focusing on the estimation of disorder prevalences and subtypes at a time when other branches of epidemiology are making progress in documenting risk factors and developing preventive interventions