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.