the trend? Assume, for convenience, that the fraction of people falsely reporting that they used drugs is fixed over time. In this case, the trend is revealed if the fraction of people falsely denying use is also fixed over time. The direction of the trend is revealed if one can rule out the possibility that the fraction of respondents falsely denying use changes enough to offset the observed trend. For instance, the reported prevalence rates for adolescents ages 12–17 in the NHSDA increased from 13.1 percent in 1991 to 18.8 percent in 1997. As long as the fraction of respondents falsely denying use did not decrease by more than 5.7 points, the actual prevalence rate increased.

Formally, the trend equals



+{P[wt+j=0, yt+j=1]–P[wt+j=1, yt+j=0]}

–{P[wt=0, yt=1]–P[wt=1, yt=0]}


The growth in prevalence rates equals the trend in reported use plus the trend in net misreporting, where net misreporting is defined as the difference between the fraction of false negative and false positive reports. Thus, the data identify trends if net misreporting is fixed over time. This assumption does not require the fraction of false negative reports to be fixed over time. Rather, any change in the fraction of false negative reports must be exactly offset by an equal change in the fraction of false positives.

For the drug use surveys, this invariance assumption seems untenable. There is no time-series evidence on inaccurate response that is directly relevant to the national household surveys. Some, however, have argued that the propensity to give valid responses is affected by social pressures that have certainly changed over time. That is, the incentive to give false negative reports may increase as drug use becomes increasingly stigmatized (Harrison, 1997). In this case, inaccurate response is likely to vary over time.

Figure D.4 displays the MTF and NHSDA reported time-series in prevalence rates along with measures of the stigma of drug use elicited from MTF 12th graders and from NHSDA respondents ages 12–17. This stigma index measures the fraction of respondents who either disapprove of illegal drug consumption (MTF) or perceive it to be harmful (NHSDA).14 The striking feature of this figure is that these prevalence rate


The specific question from MTF used to create this index is: “Do you disapprove of people (who are 18 or older) smoking marijuana occasionally?” The NHSDA question used to create the index is: “How much do you think people risk harming themselves physically or in other ways when they smoke marijuana occasionally/once a month?”

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