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While the NHSDA collects data on the most severely affected drug users, the survey design is less suited to estimate these problems. The limitations that preclude more accurate estimates are primarily the sample size, coverage, and the use of a self-report. Because heavy drug use is relatively rare in the general population, the NHSDA captures a small number of these users, resulting in a relatively large sampling error. In addition to this instability resulting from the small sample, underestimation is believed to occur because many heavy drug users may not maintain stable addresses and, if located, may not be available for an interview. Finally, as with all NHSDA respondents, heavy drug users who participate in the survey may not always report their drug use accurately during the interview.
A new estimation procedure was designed at OAS to produce improved estimates of heavy drug use (Wright, Gfroerer and Epstein 1995). This procedure uses external counts of the number of people in treatment for drug problems (from the National Drug and Alcoholism Treatment Unit Survey) and the number of arrests for non-traffic offenses (from the F.B.I.'s Uniform Crime Reports) to adjust NHSDA data. This ratio estimation procedure provides a partial adjustment that accounts for undercoverage of hard-to-reach populations and also adjusts for underreporting of drug use by survey respondents. However, it does not reduce sampling error.
Past applications of this adjustment have resulted in 40-80 percent higher estimates of past year heroin use and 20-40 percent higher estimates of frequent cocaine use. Adjustments have not yet been developed for the 1995 NHSDA, but these previous results point out the likelihood of substantial underestimation of some of these measures (unadjusted) shown in this report.
This page was last updated on June 16, 2008.
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