The study design followed the tenets of the Declaration of Helsinki for biomedical research and was approved by the Institutional Review Board of the Catholic University of Korea in Seoul, Korea.
A population-based cross-sectional study was performed in consecutive male conscripts who were aged 19 years and normally resident in Jeju during the study period (2011). Jeju is the largest island in the republic of Korea (South Korea) and is located in the south part of Korea. Jeju has a lower personal income (US $19,829, 2010) than that of Seoul (US $24,988, 2010), and has a higher proportion of farming and fishing industry (17.7%, 2010) than that of Seoul (0.2%, 2010). Because all 19-year-old males in South Korea have a legal obligation to undergo a physical examination for conscription, all 19-year-old males residing in Jeju Island were eligible for inclusion, yielding a total sample of 2805 individuals. Males exempted from military service were included in this study because they did undergo physical examinations in the process of determining exemption from military service. Exclusion criteria in this study included persons who had surgery for retinal detachment—such as vitrectomy, encircling, or buckling—because refractive errors may change after these surgeries. For subjects who had received refractive surgery, we obtained data concerning the preoperative refractive error from the hospitals in which they received surgery and included them using their hospital data.