The mean ratio of absolute anomalous intensity
               differences to their standard deviation within the
               spherical shell (defined by its diffraction limits
               _reflns_shell.d_resolution_high and
               _reflns_shell.d_resolution_low).
                    |Dano|
                 -------------
                  sigma(Dano)
               with
                 Dano        = I(+) - I(-)
                 sigma(Dano) = sqrt( sigma(I(+))2 + sigma(I(-))2 )