The sample design of the survey has changed over time, but it has always been representative of the U.S. general population age 12 and older and has always oversampled youths and young adults. By assigning the appropriate selection probabilities at the PSU, segment, and person levels, oversampling of certain subpopulations of interest is accomplished. Introductory letters are sent to sampled addresses, followed by an interviewer visit.
Interviewers attempt to conduct interviews in a private place, away from other household members.
After these answer sheets are completed, they are placed by the respondent in an envelope, which is sealed and mailed to the contractor, Research Triangle Institute, with no personal identifying information attached. Upon receipt, questionnaires are checked for critical identification and demographic data, then keyed to disk. This creates a file consisting of one record for each completed interview. Extensive within-record consistency checks and resolution of most inconsistencies and missing data are done using machine editing routines, called logical imputation. For some key variables that still have missing values after the application of logical imputation, statistical imputation is used to replace the missing data with appropriate valid response codes.