TABLE D.3 Observed Annual Trends in Prevalence Rates of Illegal Drug Use Adolescents Ages 12–17 and the Switching Threshold for Nonrespondents, National Household Survey of Drug Abuse.

Year

Observed Trend

Switching Threshold Adolescents Ages 12–17

1992

–2.7

8.1

1993

1.5

–4.5

1994

3.6

–10.8

1995

2.5

–7.5

1996

–1.3

3.9

1997

2.1

–6.3

1991–1997

5.7

17.1

drugs.10 Thus, despite considerable resources devoted to reducing misreporting in the national drug use surveys, inaccurate response remains an inherent concern. Surely some respondents fail to provide valid information about whether they consume illegal drugs.

Inaccurate reporting in drug use surveys is conceptually different from the nonresponse problem examined above. While the fraction of nonrespondents is known, the data do not reveal the fraction of respondents who give invalid responses to the questionnaire. It might be that all positive reports are invalid, in which case the usage rate may be zero. Alternatively, it might be that all negative reports are invalid, in which case the entire population may have consumed illegal drugs. Thus, to draw inferences about the fraction of users in the United States, one must impose assumptions about self-reporting errors.

To evaluate the impact of invalid response on the ability to infer levels of use, I introduce notation that distinguishes between self-reports and the truth. Let wt be the self-reported measure in period t, where wt= 1 if the respondent reported use and 0 otherwise. Let yt be the truth, where yt=1 if the respondent consumed drugs and 0 otherwise. We are interested in learning probability of use, P[yt=1]. Formally, we can relate this unobserved prevalence rate to the self-reported usage rates as follows:

P[yt=1]=P[wt=1, yt=1]+P[wt=0, yt=1]=P[wt=1]

+P[wt=0, yt=1]–P[wt=1, yt=0].

(8)

10  

While some respondents may falsely claim to consume drugs, the consensus is that false negative reports are more pervasive, even among adolescents. The principal investigators of the MTF summarize this widely held view when they state, “insofar as any reporting bias exists, we believe it to be in the direction of under-reporting. Thus, we believe our estimates to be lower than their true values” (Johnston et al., 1998a:47).



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