Add Health used a school-based design that selected 80 high schools and a paired feeder school from a stratified list of all high schools in the US in 1994. Schools were stratified by region, urbanicity, school type (public, private, parochial), ethnic mix, and size; 79% of the schools contacted agreed to participate in the study. An in-school questionnaire was administered to more than 90,000 students in grades 7-12 who attended these schools during the 1994-95 school year, and school administrators also filled out a questionnaire about the school. The in-school student questionnaire provided data on the school context, friendship networks, school activities, future expectations, and a variety of health conditions. An additional purpose of the in-school questionnaire was to identify and select special supplementary samples of individuals in rare but theoretically crucial categories. It is this aspect of the design that enabled Add Health to oversample twins and other sibling pairs according to their genetic relatedness within this nationally representative sampling frame.
