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 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.

               _refine.ls_R_factor_obs should not be confused with
               _refine.ls_R_factor_R_work; the former reports the results of a
               refinement in which all observed reflections were used, the
               latter a refinement in which a subset of the observed
               reflections were excluded from refinement for the calculation
               of a 'free' R factor. However, it would be meaningful to quote
               both values if a 'free' R factor were calculated for most of
               the refinement, but all of the observed reflections were used
               in the final rounds of refinement; such a protocol should be
               explained in _refine.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