**2.2 Participants and sampling**

The ABCD study is a multi-site longitudinal study that has recruited 11,875 children aged 9–10, from 21 cities across different states, to characterize their psychological and neurobiological development from early adolescence to early adulthood [78]. Most of the participants were recruited through schools across the 21 study sites [81]. Because of well-designed and performed sampling process, the ABCD study sample has generated a sample that although is not nationally representative, it is a balanced sample that has a strong proxy of US adolescents [81]. Thus, the ABCD sample is a close approximation of US children in terms of distribution of age, SES, ethnicity, sex, and urbanicity [81].
