ever, there is more reason to be concerned that decisions to respond truthfully, if at all, are motivated by respondents’ reluctance to report that they engage in illegal and socially unacceptable behavior. To the extent that nonresponse and inaccurate response are systematic, surveys may yield invalid inferences about illicit drug use in the United States.
In fact, it is widely thought that self-reported surveys of drug use provide downward biased measures of the fraction of users in the surveyed subpopulations (Caspar, 1992; Harrison, 1997; Mieczkowski, 1996). Nonrespondents are likely to have higher prevalence rates than those who respond. False negative responses may be extensive in a survey of illicit activity. While it is presumed that self-report surveys fail to accurately measure levels of use, they are often assumed to reveal trends. The principal investigators of MTF summarize this widely held view when they state (Johnston et al., 1998a:47–48):
To the extent that any biases remain because of limits in school and/or student participation, and to the extent that there are distortions (lack of validity) in the responses of some students, it seems very likely that such problems will exist in much the same way from one year to the next. In other words, biases in the survey will tend to be consistent from one year to another, which means that our measurement of trends should be affected very little by any such biases.
These same ideas are expressed in the popular press as well as in the academic literature. Joseph Califano, Jr., the former secretary of health, education and welfare, summarized this widely accepted view about the existing prevalence measures (Molotsky, New York Times, August 19, 1999): “These numbers understate drug use, alcohol and smoking, but statisticians will say that you get the same level of dissembling every year. As a trend, it’s probably valid.” Anglin, Caulkins, and Hser (1993:350) suggest that “making relative estimates is usually simpler than determining absolute estimates…. [I]t is easier to generate trend information…than to determine the absolute level.”
To illustrate the inferential problems that arise from nonresponse and inaccurate response, consider using the MTF and the NHSDA surveys to draw inferences on the annual prevalence of use rates for adolescents. Annual prevalence measures indicate use of marijuana, cocaine, inhalants, hallucinogens, heroin or nonmedical use of psychotherapeutics at least once during the year. Different conclusions about levels and trends might be drawn for other outcome indicators and for other subpopulations.
Figure D.1 and Table D.1 display a time-series of the fraction of adolescent users as reported in the official annual summaries of the MTF (Johnston et al., 1998b) and the NHSDA (Office of Applied Studies, 1997).