The overall mean ratio of absolute anomalous intensity
differences to their standard deviation within the
sphere defined by the diffraction limits
_reflns.d_resolution_low) and using data for which
intensities for both instances of a Friedel pair are
available for an acentric reflection.
Dano = I(+) - I(-)
sigma(Dano) = sqrt( sigma(I(+))2 + sigma(I(-))2 )