The purpose of the ECA program was to collect data on the prevalence and incidence of mental disorders and on the use of and need for services by the mentally ill in the United States. The study sites were areas that had previously been designated as Community Mental Health Center catchment areas: New Haven, Connecticut, Baltimore, Maryland, St. Louis, Missouri, Durham, North Carolina, and Los Angeles, California. Each site sampled over 3,000 community residents and 500 residents of institutions, yielding 20,861 respondents overall. The ECA incorporated two waves of personal interviews administered one year apart, as well as a brief telephone interview in between.