2.1. National Health Insurance System (NHIS)-National Sample
Cohort (NSC)
In South Korea, almost all Korean citizens are enrolled in the National
Health Insurance System (NHIS). The NHIS collects health data from
nearly 50 million of the insured subjects, including admission and
outpatient visit records, diagnoses, drug prescriptions, national health
examination data, death, and causes of death. The NHIS provides all the
above-mentioned information for research purposes. For the present
study, we used data from the NHIS-National Sample Cohort (NSC), a
specialized data set provided by NHIS that includes a stratified random
sample for the total eligible Korean population (approximately 1 million
people)11.