(Schütze, 1998) points out that single link clustering tends to place all instances into a single elongated cluster, whereas (Pedersen and Bruce, 1997) and (Purandare, 2003) show that hierarchical agglomerative clustering using average link (via McQuitty's method) fares well.
we have chosen to use average link/UPGMA as our criteria function for the agglomerative experiments.