The NCS-A is a nationally representative, face-to-face survey of 10 123 adolescents aged 13 to 18 years in the continental United States conducted between February 1, 2001, and January 30, 2004.24 The survey was administered by the professional interview staff of the Institute for Social Research at the University of Michigan, Ann Arbor. There were 197 interviewers supervised by a team of 18 experienced regional supervisors. A study manager located at the central facility in Michigan oversaw the work of the supervisors and their staff. The NCS-A was performed in a dual-frame sample that included a household subsample and a school subsample.24–27 The overall NCS-A response rate combining the 2 subsamples was 82.9%. Comparisons of sample and population distributions on US Census sociodemographic variables, and in the school sample on school characteristics, documented only minor differences, which were corrected with 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
