Modeling and Multivariate Methods - SAS

Chapter 18 Clustering Data 465
Hierarchical Clustering
The scree plot beneath the dendrogram has a point for each cluster join. The ordinate is the distance that
was bridged to join the clusters at each step. Often there is a natural break where the distance jumps up
suddenly. These breaks suggest natural cutting points to determine the number of clusters.
Open the Clustering History table to see the results shown in Figure 18.3.
Figure 18.3 Clustering History
The number of clusters begins with 32, which is the number of rows in the data table minus one. You can
see that the two closest points, Germany and Italy, are joined to reduce the number of existing clusters to 32.
They show as the first Leader and Joiner in the Clustering History table. The next two closest points are
Poland and United Kingdom, followed by China and Thailand. When Australia is joined by China in the
eighth line, China had already been joined by Thailand, making it the third cluster with three points. The
last single point to be joined to others is Afghanistan, which reduces the number of clusters from nine to
eight at that join. At the very end, a cluster of two points led by Afghanistan is joined by the rest of the
points, led by Algeria. The order of the clusters at each join is unimportant, essentially an accident of the
way the data was sorted.