Residual factor R for reflections that satisfy the resolution
               limits established by _refine.ls_d_res_high and
               _refine.ls_d_res_low and the observation limit established by
               _reflns.observed_criterion, and that were used as the test
               reflections (i.e. were excluded from 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|F~obs~ - F~calc~|
               R = ---------------------
                        sum|F~obs~|

               F~obs~  = the observed structure-factor amplitudes
               F~calc~ = the calculated structure-factor amplitudes

               sum is taken over the specified reflections