Study sources

After approval from the institutional review board at Northwestern University, cross-sectional data from 19 different population-based studies, including the 1997–2013 National Health Interview Survey (NHIS) and the 2003/4 and 2007/8 National Survey of Children’s Health (NSCH), were assessed. The specific characteristics of each survey are presented in Table S1. Briefly, both the NHIS, collected in-person by trained interviewers, and NSCH, collected over telephone, were surveys designed and overseen by the National Center for Health Statistics (NCHS), for the purpose of estimating the prevalence of health issues affecting children in the United States. In households that were selected to participate in NHIS or NSCH, one child was randomly chosen to be the subject of an interview with the caregiver, and each study year included a sample of subjects drawn independently from previous years. Inherent to each survey’s design is a complex, multistage probability sample that incorporated demographic data from the U.S. Census bureau. From this incorporated data, the NCHS created sample weights that allow for prevalence estimations that are representative of the US population of non-institutionalized children. In this study, prevalence estimates of data from either NHIS or NSCH reflect this complex weighting process. However, in pooled prevalence estimates of data from all 19 surveys, sample weights could not be combined due to differences in sampling methodology between the two survey vehicles.