Home > Encyclopedia of Drugs, Alcohol, and Addictive Behavior > National Household Survey On Drug Abuse (Nhsda)

HISTORY OF THE NHSDA

The NHSDA traces its origin to a survey conducted by the NATIONAL COMMISSION ON MARIHUANA AND DRUG ABUSE (1970-1972). The commission required baseline data on the public's beliefs, attitudes, and use of marijuana, to satisfy its charge of developing recommendations for legislation and administrative actions in helping to deal with the illicit drug problem. Through a private contractor, they conducted two surveys, in 1971 and 1972. The NATIONAL INSTITUTE ON DRUG ABUSE (NIDA) continued the survey in subsequent years (1974, 1976, 1977, 1979, 1982, 1985, 1988, 1990, and 1991) to satisfy the continuing need for current data. Starting in 1990, the survey was conducted annually. In 1992, sponsorship of the survey was transferred to the newly created SUBSTANCE ABUSE AND MENTAL HEALTH SERVICES ADMINISTRATION (SAMHSA). All the surveys were conducted by private contractors selected by the government.

Expansion of the survey took place in 1985 with the implementation of a new sample design that had larger samples of African Americans and HISPANICS (resulting in a sample size of 8,038). Further expansions took place in 1990, with the intensive sampling of the Washington D.C., metropolitan area as a part of the survey and in 1991, with the addition of five more oversampled metropolitan areas and an increase in the national sample component (for a total sample of 32,594 in 1991). The metropolitan oversampling was continued through 1993, but beginning in 1994 the survey was scaled back to a national sample of about 18,000 interviews. All surveys conducted from 1971 through 1991 were done at a particular time of year, usually spring or fall. In 1992, a continuous data collection design was implemented—with quarterly samples and January to December data collection. A major revision to the survey questionnaire was also implemented in 1994, to improve the validity and reliability of the survey estimates. That year, the NHSDA began using an improved questionnaire and estimation procedure based on a series of studies and consultations with drug survey experts and data users. Because this new methodology produces estimates that are not directly comparable to previous estimates, the 1979-1993 NHSDA estimates presented in the 1998 report were adjusted to account for the new methodology that was begun in 1994.

The 1998 NHSDA employed a sample of 25,500 persons. This sample included augmented samples in California and Arizona (4,903 and 3,869 respectively).

DESCRIPTION OF THE SURVEY METHODOLOGY

Since its inception, the NHSDA has undergone various design changes affecting primarily the sample design, as described above.

Target Population.

Prior to 1991, the NHSDA covered all persons age twelve and older living in households in the forty-eight contiguous states. Beginning in 1991, this was modified so that the survey covers the civilian noninstitutionalized population aged twelve years old and older within the fifty states. In addition to including all household residents (except persons on active MILITARY duty), it includes the residents of noninstitutional group quarters (e.g., shelters, rooming houses, dormitories) as well as residents of civilian housing on military bases. Persons excluded from the target population are those with no fixed address, residents of institutional quarters (e.g., jails and hospitals), and active-duty military personnel.

Sample Selection.

A complex multistage sample design is used to select people to be respondents in the survey. The first stage of sampling is the selection of nonoverlapping geographic primary sampling units (PSUs), consisting of counties or metropolitan areas. For the second stage of sampling, area segments (constructed from U.S. Census block groups or enumeration districts) are selected within each PSU. Field staff count and list all dwelling units within sample segments and mark their location on a map. A dwelling unit is either a housing unit, such as a house or apartment, or a group-quarters unit, such as a dormitory room or a shelter bed. From these listings, a sample of dwelling units is then selected by sampling staff, and interviewers are assigned to contact these dwelling units.

Prior to arrival at the sample dwelling unit (SDU) an introductory letter is mailed to the SDU, briefly explaining the survey and requesting participation. When the interviewer visits the SDU, a brief screening interview is conducted that involves listing all SDU members along with their basic demographic data on a screening form. The interviewer identifies which SDU member(s) will be asked to participate in the survey, based on the composition of the household. This selection process is designed to provide the necessary sample sizes for specified population groups.

Questionnaire Administration.

Interviewers control the questionnaire administration, but to enhance respondent confidentiality, drug-use questions are answered by respondents on self-administered answer sheets that are not reviewed by interviewers. As the respondent records the answer choices and completes each answer sheet, they are placed in an envelope. At the end of the interview process, all materials are sealed in this envelope by the respondent and mailed to the data-processing site with no personal identifying information attached.

Data Processing.

All questionnaires are received by mail at a data-processing site, where they are checked for critical identification and demographic data and then all data are entered onto a computer data base. Consistency checks and other editing is done, after which statistical tables showing estimates of prevalence rates for various drugs are produced. Data are generally released to the public about six months after the end of data collection. Public use data files are available one to two years after completion of data collection.

STRENGTHS AND LIMITATIONS OF THE NHSDA

Strengths.

The major strengths of the NHSDA are its size, continuity, and national representativeness. The survey has a sample large enough to allow comparisons of drug-use prevalence among many different population subgroups each year and over time. The length of the questionnaire and amount of data collected provides a rich data base for examining the characteristics of drug abusers, the relationships of drug use with many demographic and other variables, and the changing patterns of drug use over time. The methodology used, while expensive, has been extensively evaluated and found to be effective (relative to other methodologies) in eliciting valid data from respondents. Through intensive call-back procedures, participation rates in the NHSDA have been excellent. The 1998 participation rate for the screening questionnaire was 93 percent and the participation rate for the main questionnaire was 77 percent.

Limitations.

The survey does not cover certain populations likely to have heavy illicit drug use, such as the homeless and prison populations. While these missing populations, because they are small, make little difference in estimating MARIJUANA or ALCOHOL prevalence, rarer behaviors such as HEROIN or CRACK use may be severely underestimated by the NHSDA. Data validity from the survey is also in question because of the self-report methods employed and the voluntary nature of the survey.

MAJOR FINDINGS OF THE SURVEY

The NHSDA has tracked the changing nature of drug abuse since 1971. At the time of the first survey, about 10 percent of the population age twelve and older had ever used illicit drugs. This was estimated to be more than double the rate of lifetime use as of the early 1960s. In 1998, an estimated 13.6 million persons or 6.2 percent of the American population of 12 years of age or older were current illicit drug users, meaning they had used an illicit drug in the month prior to interview. The report for current use showed that more than one drug had been used by some of the total 13.6 million, with a breakdown of this figure as follows: Some 11 million reported using marijuana or HASHISH; an estimated 1.8 million cocaine; and 130,000 heroin. The rate of current use of inhalants by Americans has remained steady since 1991 (between 0.3-0.4 percent of the population). The rate of current use of HALLUCINOGENS and PRESCRIPTION DRUGS was estimated at 0.7 percent and 1.1 percent respectively in 1998. By 1998, the estimated number of persons who had tried methamphetamine in their lifetime was 4.7 million (2.1 percent of the population). Current use of illicit drugs reached a peak in 1979 when the estimate was 25 million, or 13.7 percent of the population.

All the NHSDAs conducted since 1971 have shown that marijuana is the most commonly used illicit drug, with current use at 5 percent in 1998. Marijuana initiation among youths 12-17 was at its highest level ever from 1995-1997. Current cocaine use reached a peak in 1985 at 3.0 percent, but the survey showed declines in cocaine use after 1985, to 0.7 percent in 1992. The percentage of current cocaine use did not change significantly between 1992 and 1998.

The NHSDA has shown varying rates of use in different segments of the population. The highest rates of current illicit drug use were found among young people age 18-20 (19.9 percent) in 1998. The rates of use generally decline in each successively older age group, with only 0.7 percent of persons age 50 and older reporting current illicit use.

The surveys have also shown that while illicit drug use occurs in all segments of society, prevalence rates have been greatest among males; in metropolitan areas; and among high-school dropouts. According to the 1998 report, although the rate of drug use was higher among the unemployed, most drug users were employed. The rate of current illicit drug use was also found somewhat higher among blacks (8.2 percent) than among whites (6.1 percent) and Hispanics (6.1 percent). With respect to absolute numbers in the 1998 report, however, most current illicit drug users were white.

The increase in marijuana use among youths age 12-17 has important implications for substance abuse prevention and treatment efforts. In terms of prevention, there is an obvious need to focus immediate attention on children and adolescents. In the long run, the expanding pool of young people using illicit drugs will probably result in continuing pressure on the substance abuse treatment system in future years, as many new drug users progress to addiction and require intervention.

(SEE ALSO: Drug Abuse Warning Network; ; High School Senior Survey; )

JOSEPH C. GFROERER

REVISED BY PATRICIA OHLENROTH