Similarly, a silhouette analysis was performed for the four 10-year sub-periods in order to determine the number of K-means clustering for each sub-period. In doing so, all the silhouettes were visually examined based on different distance metrics as previously described in the analysis of the full-time period (1980–2020). Our analysis indicates that the cosine distance method gave the best result for the three sub-periods, including 1980–1989, 1990–1999, and 2010–2020. Only for the 2000–2009 period does the correlation distance method give the optimum number of clusters (Table 2).