Abstract
Clinical epidemiology is the science which can answer many questions in clinical medicine. Clinical epidemiology can quantitatively evaluate many problems and try to optimize medical technology. Among several ways of clinical research, case-control study, cohort study, and randomized controlled trial can test hypothesis with their own strength and weakness. The problems in these research are bias, confounding, and chance which we can manage with several ingenious methods. Randomized controlled trial which has the strongest power to suggest cause-effect relationship has many issues when we interpret or design randomized controlled trial. When we plan to research in clinical medicine, epidemiologist should be engaged early at design-phase of the study.
