poststratification weighting. The recruitment and consent procedures were approved by the Human Subjects Committees of both Harvard Medical School and the University of Michigan. On making in-person contact, the interviewer answered questions before obtaining written informed consent from the parent and then written informed assent from the adolescent. Once the survey was completed, cases were weighted for variation in within-household probability of selection (in the household subsample) and for residual discrepancies between the sample and the US population on the basis of sociodemographic and geographic variables. These weighting procedures are discussed in more detail else-where.26,27 Sociodemographic variables examined in this report include age (in years), sex, and self-described race/ ethnicity. Approximately half the sample was male (51.3%) and the mean age was 15.2 years, with a larger proportion of adolescents aged 13 to 14 years (36.2%) and approximately equal distributions of adolescents aged 15 to 16 and 17 to 18 years. The sample was composed of 65.6% non-Hispanic whites, 15.1% non-Hispanic blacks, and 14.4% Hispanics.
