# File lib/ai4r/clusterers/weighted_average_linkage.rb, line 51
      def linkage_distance(cx, ci, cj)
        ni = @index_clusters[ci].length
        nj = @index_clusters[cj].length
        (1.0 * ni * read_distance_matrix(cx, ci)+
          nj * read_distance_matrix(cx, cj))/(ni+nj)
      end