Adolescent and close friend use of alcohol and marijuana were assessed with the Alcohol and Drug Use Questionnaire (Johnston, O'Malley, & Bachman, 1987), a self-report measure that includes items assessing the frequency of adolescent use of alcohol and/or marijuana in the past 30 days. It was completed independently by both target teens and their closest friend at ages 15 and again by the target teen at age 16. This measure is based on the “Monitoring the Future” surveys (Johnston, O'Malley, & Bachman, 1987). Johnston and colleagues (1987) found high reliability from year to year and consistency between related measures within the same questionnaire administration. Construct validity in their research was demonstrated as self-reported substance use was related to attitudes, beliefs, and related behaviors and under-reporting appeared to be minimal.
