Main steps involved in the type of cluster analysis used in this paper. (a) 11 data sets are characterized with 22 numerical values taken by two statistical descriptors. (b) These data sets can be represented in a two-dimensional space (a plane) as there are only two descriptors. (c) Hierarchical cluster analysis carried out using Ward linkage gives rise to this dendrogram. Data sets and clusters merge into progressively larger groups as the cluster height considered is increased. (d) Specific clusters corresponding to merging nodes can be isolated using one or two height levels. Only data sets 1–7 are selected in this example because only one merging node is included between levels Lt and Lb.