Residual factor R for reflections that satisfy the resolution
               limits established by _refine_hist.d_res_high and
               _refine_hist.d_res_low and the observation limit
               established by _reflns.observed_criterion, and that were used
               as the working reflections (i.e. were included in the refinement)
               when the refinement included the calculation of a 'free'
               R factor. Details of how reflections were assigned to the
               working and test sets are given in _reflns.R_free_details.
                   sum|Fobs - Fcalc|
               R = ---------------------
                        sum|Fobs|
               Fobs  = the observed structure-factor amplitudes
               Fcalc = the calculated structure-factor amplitudes
               sum is taken over the specified reflections