specified purity. Because quantity and purity information are needed to estimate both prices and consumption, the committee has concluded that the acquisition of price and consumption data is usefully thought of as different aspects of the same problem. A method for acquiring information on the price, quantity, and purity of retail drug purchases is outlined in the section below on drug prices.

Population Coverage and Sample Size Issues
What the NHSDA and MTF Can and Cannot Reveal About Drug Use

The NHSDA and MTF are national probability surveys of particular segments of the U.S. population. Setting aside the response problems discussed later in this chapter, the sample data can be used to draw inferences about the surveyed populations. MTF does not survey high school dropouts. The NHSDA excludes about 2 percent of the population age 12 and older, including active military personnel, persons living in institutional group quarters (e.g., prisons and residential drug treatment centers) and homeless people not living in shelters.

Incomplete coverage is a problem if one wants to draw inferences for the general population or the excluded subpopulations, rather than the surveyed subpopulations. The groups not covered by the NHSDA and MTF sample designs may be particularly important for monitoring drug use in the United States. High school dropouts, homeless people, and people in institutions may exhibit substantially higher rates of drug use than the general population.4 These high-risk “hidden populations” may contribute disproportionately to drug use in general and the use of more stigmatized drugs in particular (U.S. General Accounting Office, 1993).

The NHSDA and MTF data alone cannot be used to draw inferences about groups that are not surveyed. Similarly, the data cannot provide reliable estimates for subpopulations that are surveyed but for whom the effective sample sizes are small.5 Thus, these surveys cannot be used to draw precise inferences about variation in drug use across important

4  

Also excluded are active military personnel who are less likely to use drugs. The Worldwide Survey of Substance Abuse and Health Behaviors among Military Personnel covers active military personnel (see, for example, Bray et al., 1995; Bachman et al., 1999; Mehay and Pacula, 1999). Chapter 7 provides further discussion.

5  

As in many large-scale national surveys, the NHSDA and MTF use complex sample designs that first sample clusters and then respondents within clusters. In clustered samples, the number of observations is not tantamount to the effective sample size. These survey design effects should be accounted for when evaluating the precision of an estimator. For further details of the effective sample sizes in the national drug use surveys, see Gfroerer et al. (1997b).



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