survey relies on self-reports, and the stigma associated with drug injection may contribute to greater denial on the part of those who do participate.

Given the fact that the NHSDA is widely acknowledged to underrepresent injection drug users, it is surprising to note the magnitude of the subpopulation it has uncovered.2 The 1991 survey revealed an estimated 3,768,000 people who have used a needle to inject a drug at some time in their lives and 1,083,000 who have done so in the past year. The 1992 estimated figures were much lower: 2,984,000 for lifetime prevalence and 659,000 for the past year. With regard to age, in 1991, 29 percent of those who reported having injected drugs in the past year were between the ages of 12 and 25 (Table 2.1). In 1992, that age group represented 44 percent of those who had reported injecting drugs in the past year.

Because the populations sampled in these 2 years were highly similar, the observed drop in lifetime prevalence and observed fluctuations by other demographic characteristics cannot be credibly attributed to the relatively minor changes in the sampling frame from one year to the next. As a consequence, this anomaly helps to underscore the imprecision with which the NHSDA is able to estimate numbers of injection drug users.

The large fluctuations in the NHSDA estimates are due in part to the nature of the targeted population. That is, the survey is designed to capture drug use among households nationwide, not specific population subgroups such as injection drug users. As a result, there are too few members of certain specific subgroups (e.g., injection drug users) to allow for the computation of reliable estimates. This is reflected in the large standard errors associated with the estimated number of injection drug users reported above.

Washington DC Metropolitan Area Drug Study

The Washington DC Metropolitan Area Drug Study (DC*MADS) was funded by the National Institute on Drug Abuse (1993, 1994b, 1994c) in an attempt to examine the nature and extent of drug abuse among all types of people residing in a single metropolitan area, with a special focus on populations that were underrepresented or unrepresented in the NHSDA (National Institute on Drug Abuse, 1994b). These special populations, including homeless people, transients, and institutionalized individuals, represent people who tend to be at risk for drug abuse.

Of particular relevance to the topic of this report, DC*MADS estimated that adding homeless and institutionalized populations to the 1991 NHSDA population of past-year injection drug use in the D.C. MSA would have increased the estimate of currently active injection drug users from 0.20 to 0.30 percent, an increase of one-third. The number of injection drug users among the population of households was estimated at 5,987; including the homeless, transient, and institutionalized populations, the number was 8,740.

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