Add Health obtained rosters of all enrolled students in each school. From the union of students on school rosters and students not on rosters who completed in-school questionnaires, a gender- and grade-stratified core sample of 200 adolescents was selected from each school pair for a 90-minute in-home interview as part of the Wave I interview. The core in-home sample is essentially self-weighting, and provides a nationally-representative sample of 12,105 American adolescents in grades 7 to 12. Twins and other sibling pairs occur naturally in the core in-home sample proportional to their representation in the general population of adolescents in grades 7 to 12 in the United States in 1995. However, to increase the potential number of twins for genetic analysis, twins and other sibling pairs were oversampled for the in-home sample based on responses from the in-school survey.
