# File lib/ai4r/clusterers/ward_linkage.rb, line 52
      def linkage_distance(cx, ci, cj)
        ni = @index_clusters[ci].length
        nj = @index_clusters[cj].length
        nx = @index_clusters[cx].length
        ( ( ( 1.0* (ni+nx) * read_distance_matrix(cx, ci) ) +
            ( 1.0* (nj+nx) * read_distance_matrix(cx, cj) ) ) / (ni + nj + nx)  -
            ( 1.0 * nx * read_distance_matrix(ci, cj) / (ni+nj)**2 ) ) 
      end