Mental retardation exists as a category of thought, a way of classifying people. Traditionally, two models have been used in making such classifications: the pathological model of medical practitioners, and the statistical model of psychologists and other behavioral scientists. Both regard mental retardation as individual pathology characterized by symptoms that manifest themselves in comparable forms in all sociocultural segments of American society. In the clinical approach, a set of operations are developed for evaluating the objects of experience along those dimensions which have been selected as the basis for classification. In the social system approach, the meaning of the concept "mental retardation" rests in understanding the use to which the concept is put, e.g. how the concept is used to sort out, classify, and label people. When we comprehend this sorting and labeling process and the outcomes of the process in terms of who is and who is not labeled, then we comprehend the meaning of mental retardation in an American community. In the social system epidemiology, we began in the empirical world of Riverside and attempted to locate all these persons who were holding the status and playing the role of mental retardate in any of the social systems of the community. Beginning at the level of experience and analyze the characteristics which persons labeled as mentally retarded by different types of social systems had in common and those characteristics on which they differed, we moved, inductively, from the empirical world up the abstraction ladder to the mental construct of the meaning of mental retardation in the city of Riverside. (Author/JM)