Silhouette analysis was performed for the 1980–2020 period encompassing 41 years of data, indicating that the cosine distance method yielded the optimum result for determining the cluster members (Table 1). The analysis shows that when the n_clusters are equal to 3 and 4, all the plots portray similar thickness, and hence, similar sizes as also confirmed from the labelled scatter plot (Figure 4). Other silhouettes showed more ambiguous and misleading results in deciding the proper number of clusters. With the cosine distance method, the average silhouette score and the number of negative values are 0.287 and 3, respectively, for n-clusters which equal 5. In other words, the optimum cluster number is defined based on the minimum negative values and the maximum average silhouette value.