Author contributions
Ellen AR Kim conceived the study design, supervised data collection and analysis and is responsible for the overall content as guarantor. ML Chung conceived the study design, collected data, analysis and interpretation of data, and produced the first draft of the paper. HS Sung helped plan the study and supervised the analysis and interpretation of the data. ES Jung collected data, analysis and interpretation of data. BS Lee collected data, analysis and interpretation of data. KS Kim conceived the study design, collected data, analysis and interpretation of data. All authors contributed to successive drafts, and have approved the final draft.