2005) and OD (Gelernter et al., 2006). The approach used a range of cocaine or opioid use behaviors to which k-means (Seber, 1984), a classic non-hierarchical clustering method, was applied in combination with a hierarchical clustering method (Hastie et al., 2001) based on Ward’s aggregation criterion. However, because the k-means method is an iterative procedure initialized with randomly chosen cluster centers, it is sensitive to outliers, with different initialization known to yield different clusters. Cross tagging multiple k-means mitigates this problem, but does not guarantee the creation of replicable subgroups.
