Data source and study population

Data were obtained from the last eight cohorts (2006–13) of NSDUH, an ongoing cross-sectional survey of non-institutionalized individuals in the 50 US states and the District of Columbia [26], with different participants sampled each year and therefore independent of each other. NSDUH is a nationally representative probability sample of populations living in households obtained through four stages. Census tracts were first selected within each state, then segments in each tract, then dwelling, and then respondents were selected for the final sample each year.

Surveys were administered via computer-assisted interviewing (CAI) conducted by an interviewer and audio computer-assisted self-interviewing (ACASI). Sampling weights were provided by NSDUH to address unit- and individual-level non-response. Weights were adjusted to ensure that estimates were consistent with estimates provided by the US Census Bureau. Further information on sampling and survey techniques can be found elsewhere [26]. The weighted interview response rates for 2006–13 NSDUH ranged from 71.7 to 75.7%.