Z. . Syahputri, Sutarman, and Machrani Adi Putri Siregar, “Determining The Optimal Number of K-Means Clusters Using The Calinski Harabasz Index and Krzanowski and Lai Index Methods for Groupsing Flood Prone Areas In North Sumatra”, SinkrOn, vol. 8, no. 1, pp. 571-580, Jan. 2024.