Several methods are used to minimize and measure error in the NIMH Epidemiologic Catchment Area program. Sampling methods involve the inclusion of group quarters such as prisons, nursing homes, and mental hospitals in the sample frame and the use of probability sampling throughout. Interviewing, methods include use of identical diagnostic interview protocols, centralized training of interview supervisors, standard instructions to interviewers, and reinterview of a subsample by clinicians. In the area of completion, the methods include a 75% to 80% respondent completion rate, a 95% or greater completion rate for individual questions, use of informant interviews where necessary, and statistical adjustments to correct for low completion rates in some subgroups. Analytic methods include use of a computerized diagnostic algorithm, common estimation formulas on identically formatted data files, and estimation of exact variances that take account of the multistage sample design.