Residual factor R(I) 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, calculated on the estimated
               reflection intensities.
               This is most often calculated in Rietveld refinements against
               powder data, where it is referred to as RB or RBragg.
                      sum|Iobs - Icalc|
               R(I) = ---------------------
                           sum|Iobs|
               Iobs  = the net observed intensities
               Icalc = the net calculated intensities
               sum is taken over the specified reflections