The correlation coefficient within the spherical shell
               (defined by its diffraction limits
               _reflns_shell.d_resolution_high and
               _reflns_shell.d_resolution_low) between two randomly
               chosen half-sets of anomalous intensity differences,
               I(+)-I(-) for anomalous data, i.e. data for which
               intensities for both instances of a Friedel pair are
               available for an acentric reflection.