The National Household Survey on Drug Abuse (NHSDA) is the primary source of statistical information on the use of illegal drugs by the population of the United States. Conducted periodically by the federal government since 1971, the survey collects data by administering questionnaires to a scientifically selected sample of persons age twelve and older living in the nation. The primary purpose of the survey is to estimate the prevalence of illegal drug use (i.e., the number of people using illegal drugs) in the United States, and to monitor changes in prevalence over time.

Legal drugs, such as ALCOHOL and TOBACCO, are also covered by the survey. Prevalence rates (the percentage of the population using any type of drug) for various population subgroups and for various types of drugs are generated from the survey data; these rates...