To assess the changing lifestyles, values, and preferences of American youth, measures of drug use, interrelationships among such measures, and their relationships to other variables were investigated in a nationwide survey of 1978 high school seniors. Students reported their use of cigarettes, alcohol, marijuana and 10 other categories of drugs during their lifetime, the past year, and the past 30 days. The data revealed that reported use during the past 30 days was systematically higher than predictions based on reported use during the past year, suggesting that self-reports for use during the past year substantially underestimated the frequency of use. Composite measures of drug use were subsequently developed to combine information about lifetime, annual, and 30-day use. The results indicate that the composite measures are well-suited for correlational analyses with a variety of other dimensions and can generally be used in place of the set of three separate measures (lifetime, annual, monthly) for each drug. (Author/KMF)