FIGURE D.1 Annual prevalence rate of use of an illegal drug for people ages 12– 17, 1975–1999, National Household Survey of Drug Abuse (NHSDA) and Monitoring the Future (MTF). Note: Annual prevalence measures indicate use of marijuana, cocaine, inhalants, hallucinogens, heroin or nonmedical use of psychotherapeutics at least once during the year.

SOURCES: LD Johnston, PM O’Malley, JG Bachman. (Dec. 1998). Drug use by American young people begins to turn downward. University of Michigan News and Information Services: Ann Arbor, MI. [On_line]. Tables 1b and 3. Available: www.isr.umich.edu/src/mtf; accessed 11/16/99.

Data from the MTF imply that annual prevalence rates for students in 12th grade increased from 29 percent in 1991 to 42 percent in 1997. Data from the NHSDA indicate that the annual prevalence rates for adolescents ages 12–17 increased from 13 percent in 1991 to 19 percent in 1997. The level estimates from the two surveys differ, with those from the MTF being more than twice those from the NHSDA.² Still, the trends across surveys are generally consistent. Both series suggest that from 1991 to 1997, the fraction of teenagers using drugs increased by nearly 50 percent. Does the congruence in the NHSDA and MTF series for adolescents imply that both surveys identify the trends, if not the levels, or does it merely