 The concept of big data goes far beyond the data type and includes the aspects of data analysis, such as hypothesis generating, rather than hypothesis testing. Big data focuses on temporal stability of the association rather than on causal relationship, and underlying probability distribution assumptions are frequently not required. Medical big data, as material to be analyzed, has various features that are distinct from big data of other disciplines. This article was authored by Chung Ho Lee and Hyeong Jin Yoon.