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.
               _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|Fobs - Fcalc|
               R = ---------------------
                        sum|Fobs|
               Fobs  = the observed structure factor amplitudes
               Fcalc = the calculated structure factor amplitudes
               sum is taken over the specified reflections