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