The NHSDA is an ongoing survey of the civilian noninstitutionalized population age 12 and older in the U.S. It covers civilians living in households (including on military bases) and persons living in some group quarters such as homeless shelters and college dormitories. Active military personnel are excluded.
Conducted under contract, the sample is an area probability sample of about 120 PSUs, 2,000 segments, and 55,000 households per year. Household screenings at sample households are done to select sample persons within households (either one or none, and occasionally two). About 18,000 interviews are conducted each year, and the sample is allocated equally to the four calendar quarters, resulting in essentially continuous data collection in every PSU. Sampling rates of locations and of sample persons are controlled to accomplish oversampling of blacks and Hispanics, and also to oversample populations with elevated drug use rates (e.g., 12-34 year olds and cigarette smokers) to improve precision for rare drug use behaviors.
The survey is voluntary and no monetary incentives are offered. Response rates have typically been around 95 percent for screening and 80 percent for interviewing. Interviews are done anonymously and confidentially. No names are collected, and responses to sensitive questions are obtained using self-administered answer sheets that are not seen by interviewers. Other parts of the interview, such as demographic and health status questions, are interviewer administered.
The survey has undergone design changes regularly in the past, primarily in the sample design. For example, prior to 1990 the survey was conducted every 2-3 years with a sample of less than 10,000 per year. During 1991-93, six large metropolitan areas were oversampled, increasing the sample to 30,000 per year. In 1994-6, the oversample was eliminated resulting in a sample size of 18,000 per year.
The questionnaire has also changed. In 1994, SAMHSA implemented a newly designed questionnaire that incorporated a core-module structure to enhance trend measurement while encouraging outside agencies to sponsor modules of interest. The questionnaire was developed as a result of a series of methodological studies and field tests conducted by NIDA and SAMHSA during 1989-93. Other modifications that have been considered for future NHSDAs include (1) a modification to the sample design to improve estimation of mental health conditions, (2) incorporation of computerized interviewing methods such as audio-CASI, and (3) expansion of PSU coverage to enhance State-level estimation of substance abuse.