The sample design of the NHIS follows a multistage area probability design, adjusting for non-response and allowing for nationally representative sampling of households and individuals. This sample design includes under-represented groups. The NHIS questionnaire is divided into 4 cores: Household Composition core, Family core, Sample Child core, and Sample Adult core. The Household Composition core includes basic and relationship information about all individuals in the household. The Family Core file includes socio-demographic characteristics, health insurance coverage, basic indicators of health status, injuries, activity limitations, and access to and utilization of health care services separately for each family in the household. A random child and adult from each family are selected to gather more detailed information about them, constituting the Sample Child and the Sample Adult cores, respectively. In our study, we used the Sample Adult core files with variables supplemented from the other cores. The Household response rates ranged from 89.6% in 2002 to 79.5% in 2011. All survey participants provided informed consent before participation in the survey. This study received exemption from Yale University Institutional Review Board Committee because NHIS data are publicly available and de-identified.

The NHIS Linked Mortality files include data from all surveys between 1985 and 