The silhouette analysis was again performed to obtain an accurate number of K-means clustering for the eight 5-year sub-periods in a similar way used for the 10-year sub-periods and the full-time period. Our analysis indicates that the cosine distance method gave the best result for the six 5-year sub-periods, including 1980–1984, 1985–1989,1990–1994, 1995–1999, 2000–2004, and 2015–2020 (Table 3). For the 2005–2009 and 2010–2014 sub-periods, however, the correlation distance method identified the cluster numbers more accurately.