# File lib/ai4r/clusterers/centroid_linkage.rb, line 55
      def linkage_distance(cx, ci, cj)
        ni = @index_clusters[ci].length
        nj = @index_clusters[cj].length
        ( ni * read_distance_matrix(cx, ci) +
          nj * read_distance_matrix(cx, cj) -
         1.0 * ni * nj * read_distance_matrix(ci, cj) / (ni+nj)) / (ni+nj)
      end