R_{factor} (or R_{work})The common crystallographic
R-factor (most common measure for the quality of a crystal structure, or correctness of a model structure) is:where
h,k,l are the reciprocal lattice points of the crystal, F and _{obs}F are the observed and calculated structure factor amplitudes, and _{calc}k is a scale factor. "The R-factor measures the average discrepancy between the scattering amplitudes Fc(hkl) calculated from the refined model and those measured experimentally (Fo(hkl))" Matthews BW (2009) Protein Sci.
Free R-Factor (R_{free})hkl⊂T means all reflections belonging to test set T of unique reflections. Refinement is carried out with the remaining reflections (the working set). R_{free }
"A test set of reflections is set aside from the working set and the progress of the refinement is monitored by the calculation of a free R factor which is based only on the excluded reflections".is used in cross validation during the refinement of macromolecular structures using X-ray data, its advantage being that it is unbiased by the refinement process. Ref.
Brunger, Free R value: a novel statistical quantity for assessing the accuracy of crystal structures, Nature (1992) vol. 355 (6359) pp. 472-5 For Tables in manuscripts use the following: R_{work} = Σ||F_{(obs)}|- |F_{(calc)}||/Σ|F_{(obs)}|R_{free} = as for Rwork, but calculated for 5.0% of the total reflections that were chosen at random and omitted from refinementThe importance of R-factor, from: http://ucxray.berkeley.edu/~jamesh/movies/ |

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