1998 National Household Survey on Drug Abuse

Summary of NHSDA Methodology

Revised Methodology and Adjustment of 1979-93 Estimates

Format of the Report and Explanation of Tables

Other NHSDA Reports

1. INTRODUCTION

Summary of NHSDA Methodology

The National Household Survey on Drug Abuse is the primary source of statistical information on the use of illegal drugs by the United States population. Conducted by the Federal Government since 1971, the survey collects data by administering questionnaires to a representative sample of the population at their place of residence. Since October 1, 1992, the survey has been sponsored by the Substance Abuse and Mental Health Services Administration (SAMHSA).

The survey covers residents of households, noninstitutional group quarters (e.g., shelters, rooming houses, dormitories), and civilians living on military bases. Persons excluded from the survey include homeless people who do not use shelters, active military personnel, and residents of institutional group quarters, such as jails and hospitals. Appendix 3 describes surveys that cover populations not included in the NHSDA sampling frame.

The 1998 NHSDA employed a multistage area probability sample which resulted in interviews with 25,500 persons. This survey was conducted from January through December 1998. Response rates for household screening and for interviewing were 93.0 percent and 77.0 percent, respectively. The sample design oversampled blacks, Hispanics, and young people, to improve the accuracy of estimates for those populations. In addition, residents of Arizona and California were oversampled to provide direct survey estimates for these state populations. This expansion of the sample was initiated in April, 1997 to measure the impact of propositions related to illicit drugs that were passed by voters in these States in 1996.

The household interview takes an average of about one hour to complete and uses self-administered answer sheets for the most sensitive portions of the interview. Use of these self-administered answer sheets is designed to maximize honest reporting of illicit drug use and other sensitive behaviors. The self-administered answer sheet includes questions on the recency and frequency of use of various licit and illicit drugs, opinions about drugs, problems associated with drug use, perceived need and demand for drug abuse treatment, and drug abuse treatment experience. Also collected in interviewer-administered portions of the 1998 questionnaire are data on demographic characteristics, employment, education, income, health status, health insurance, utilization of services, and access to health care.

A major change in the NHSDA design is being implemented during 1999. This change will significantly affect the results that will be reported next year. Beginning in 1999, data are being collected using computer assisted interviewing, including audio computer assisted self interviewing in place of the paper answer sheets used in 1998 and earlier. In addition, the NHSDA sample was expanded in 1999 to conduct 70,000 interviews per year, with the capability of providing state-level estimates of substance use prevalence.

Revised Methodology and Adjustment of 1979-93 Estimates

SAMHSA has allocated substantial resources to improve the NHSDA measurement of substance use and related issues for use in policymaking. A series of studies was conducted during 1988-1992 to evaluate the survey methodology (Turner, Lessler, and Gfroerer 1992). These studies identified a number of potential improvements to the NHSDA questionnaire. Based on these studies, and consultations with drug survey researchers and data users, an improved instrument was developed, tested, and fielded in 1994.

When the new questionnaire was introduced in 1994, a supplemental sample was selected for use with the old methodology (i.e., identical to 1993). This provided the capability to assess the impact of the new questionnaire and to measure the effects of the change in methodology. Analyses of the 1994 data have shown that the new methodology had a minimal effect on some estimates, but the effect on others was substantial. A separate SAMHSA report provides details on the development of the new questionnaire and the impact of the new methodology on substance use estimates (SAMHSA 1996a).

Because of the change in methodology in 1994, many of the estimates from the 1993 and earlier NHSDAs are not directly comparable to estimates from the 1994 and later NHSDAs. Since it is important to describe long-term trends in drug use accurately, an adjustment procedure was developed and applied to the pre-1994 estimates. This adjustment uses the 1994 supplemental sample to estimate the magnitude of the impact of the new methodology for each drug category. A description of the adjustment method is given in Appendix 2.

Readers need to be aware that all 1979-93 data shown in this report are different from previously published NHSDA estimates for 1979-93. Because the adjustments were developed from sample survey data, they are subject to sampling error and, therefore, may in some cases introduce additional variation into trends. This is particularly true for estimates of rare behaviors and for small subgroups.

Format of the Report and Explanation of Tables

Summaries of the 1998 NHSDA results are presented for several categories of drugs. For each drug category, recent trends and differences in use among population subgroups are described. An analysis of trends in the initiation of substance use and an analysis of data on perceived risk of harm and other measures are contained in sections following the drug use prevalence analyses. Estimates of substance dependence and treatment and some results from the California and Arizona supplemental samples follow. A discussion of the NHSDA findings is given at the end of the report. Technical appendices 1, 2 and 3 provide more detail on the NHSDA methodology, limitations of the data, and other sources of data. Appendix 4 provides a list of references related to the NHSDA, other substance abuse surveys, and survey methods. Detailed tabulations of NHSDA data are provided in Appendix 5.

The tables and analyses focus primarily on recent trends, from 1997 to 1998. Long term trends are also presented and discussed, but due to the limitations of the procedure used to adjust for the differences between the pre- and post-1994 methodology, it is not possible to analyze these data in as much detail. As indicated in the tables, statistical significance testing was done for comparisons between 1998 and prior years. Significance levels are indicated in the tables, and allchanges described in the text as increases or decreases were tested and found to be significant at least at the .05 level, unless otherwise stated.

Tables and text present prevalence measures in terms of both the number of drug users and the rate of drug use in the population. Tables show estimates of drug use prevalence in lifetime (i.e., ever used), past year, and past month. The analysis focuses primarily on past month use, which is also referred to as "current use."

Data are presented for three major racial/ethnic groups: whites, blacks, and Hispanics. A fourth category, "Other," includes Asian and Pacific Islanders, American Indians and Alaskan Natives, and other groups. It should be noted that the category "white" includes only non-Hispanic whites, the category "black" includes only non-Hispanic blacks, and the category "Hispanic" includes Hispanics of any race.

Data are also presented for four U.S. geographic regions and nine geographic divisions within these regions. These regions and divisions include the following groups of States:
 

Northeast Region - New England Division: Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut; Mid-Atlantic Division: New York, New Jersey, Pennsylvania.
 

North Central Region - East North Central Division: Wisconsin, Illinois, Michigan, Indiana, Ohio; West North Central Division: North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri.
South Region - South Atlantic Division: West Virginia, Virginia, Maryland, Delaware, District of Columbia, North Carolina, South Carolina, Georgia, Florida; East South Central Division: Mississippi, Tennessee, Kentucky, Alabama; West South Central Division: Texas, Oklahoma, Arkansas, Louisiana.
West Region - Mountain Division: Idaho, Nevada, Arizona, New Mexico, Utah, Colorado, Wyoming, Montana; Pacific Division: California, Oregon, Washington, Hawaii, Alaska.
 

The tables also present data by population density. For this variable, large metropolitan areas are defined as Metropolitan Statistical Areas (MSAs) with a population of 1 million or more. Small metropolitan areas are MSAs with a population of less than 1 million. Nonmetropolitan areas are areas outside of MSAs. For some tables, small metropolitan areas are further classified as having either less than or greater than 250,000 population, and nonmetropolitan areas are classified as either urban or rural. In this report, all rural estimates are based on only nonmetropolitan rural areas. Tables showing data for the more detailed population density categories and the geographic divisions are based on the combined 1997 and 1998 NHSDA data sets, to ensure large enough sample sizes for these categories. For 1993 and later estimates, 1990 Census data and 1990 MSA classifications were used to determine population density. For 1992 estimates, 1990 Census counts and 1984 MSA classifications were used.

Other than presenting results by age group and other basic demographic characteristics, no attempt is made in this report to control for potentially confounding factors that might help explain the observed differences. This point is particularly salient with respect to race/ethnicity,which tends to be highly associated with socioeconomic characteristics. The cross-sectional nature of the data limits the capability to infer causal relationships. Nevertheless, the data presented in this report are useful for indicating demographic subgroups with relatively high (or low) rates of drug use, regardless of what the underlying reasons for those differences might be.

Other NHSDA Reports

Two other reports are produced from the NHSDA data each year. "Population Estimates" contains additional detailed tabulations of 1998 data and is released along with this report. "Main Findings" for 1998 contains more comprehensive analyses and discussion of the results, including trends, and more detail on the survey methodology. "Main Findings" will be released early in 2000.

The Office of Applied Studies has also conducted more specialized, in-depth analyses using NHSDA data on specific substance abuse issues (see list of references in Appendix 4). Recent studies either completed or in progress include:
 

• The Relationship Between Mental Health and Substance Abuse Among Adolescents
• Substance Use and Mental Health Characteristics by Employment Status
• Worker Drug Use and Workplace Issues, 1994 and 1997
• Patterns of Alcohol Use Among Adolescents and Associations with Emotional and Behavioral Problems
• Substance Use Among Adults on Probation
• Substance Use Among School Dropouts
• Risk and Protective Factors for Adolescent Drug Use
• Seasonability of Substance Use

 
A complete listing of previously published reports from the NHSDA and other data sources is available from the Office of Applied Studies. Many of these reports are also available through the Internet (see Acknowledgments page). In addition, OAS makes public use data files available to researchers through the Substance Abuse and Mental Health Data Archive (www.icpsr.umich.edu/samhda). Currently, files are available from the 1979-1997 NHSDAs. The 1998 public use file will be available in early 2000. Secondary analysis of these data can be supported through grants awarded by the Division of Epidemiology and Prevention Research, National Institute on Drug Abuse.

This page was last updated on June 01, 2008.

SAMHSA, an agency in the Department of Health and Human Services, is the Federal Government's lead agency for improving the quality and availability of substance abuse prevention, addiction treatment, and mental health services in the United States.