1997), people in high-risk neighborhoods (Fendrich et al., 1999), and other settings. See Harrison and Hughes (1997) for a review of the literature.

Despite this literature, little is known about misreporting in the NHSDA and MTF. Existing validation studies have largely been conducted on samples of people who have much higher rates of drug use than the general population. Respondents are usually not randomly sampled from some known population. The response rates to these surveys are often quite low.

A few studies have attempted to evaluate misreporting in broad-based representative samples. However, lacking direct evidence on misreporting in the national probability surveys, these studies make strong, unverifiable assumptions to infer validity rates. Biemer and Witt (1996) analyzed misreporting in the NHSDA under the assumption that smoking tobacco is positively related to illegal drug use and independent of valid reporting. They found false negative rates (that is, the fraction of users who claim to have abstained) in the NHSDA that vary between 0 and 9 percent. Fendrich and Vaughn (1994) evaluated denial rates using panel data on illegal drug use from the National Longitudinal Survey of Youth (NLSY), a nationally representative sample of individuals who were ages 14 to 21 in the base year of 1979. Of the respondents to the 1984 survey who claimed to have ever used cocaine, nearly 20 percent denied use and 40 percent reported less frequent lifetime use in the 1988 follow-up. Of those claiming to have ever used marijuana in 1984, 12 percent later denied use and just over 30 percent report less lifetime use. These logical inconsistencies in the data are informative about validity only under the assumption that the original 1984 responses are correct.

Both of these studies require unsubstantiated assumptions to draw conclusions about validity. Arguably, smokers and nonsmokers may have different reactions to stigma and thus may respond differently to questions about illegal behavior. Arguably, the self-reports in the 1984 National Longitudinal Survey of Youth are not all valid. Thus, neither study can be used to draw strong conclusions about validity rates.

Still, several broad conclusions about misreporting have been drawn. At the most basic level, there appears to be consistent evidence that some respondents misreport their drug use behavior. More specifically, valid self-reporting of drug use appears to depend on the timing of the event and the social desirability of the drug. Recent use may be subject to higher rates of bias. Misreporting rates may be higher for stigmatized drugs, such as cocaine, than for marijuana. False negative reports seem to increase as drug use becomes increasingly stigmatized. The fraction of false negative reports appears to exceed the fraction of false positive reports, although these differences vary by cohorts. Finally, the validity rates can

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