Social epidemiological studies provide robust quantitative descriptions of health inequity. In particular, nationally representative data has exceptionally strong external validity and thus can provide substantive evidence for policy and intervention development. Korea National Health and Nutrition Examination Survey is a publicly available nationally representative dataset of South Korean adults. When using Korea National Health and Nutrition Examination Survey, multiple factors require researchers’ careful consideration. This case study describes behind-the-scene decision-making processes for data analyses of our study on how age moderates the effect of socioeconomic status on physical activity level among South Korean adults.