R-factor (crystallography)

In crystallography, the R-factor (sometimes called residual factor or reliability factor) is a measure of the agreement between the crystallographic model and the experimental X-ray diffraction data. It is defined by the following equation:

$$R = \frac{ \sum{||F_{obs}| - |F_{calc}|| } }{ \sum{ |F_{obs}|}}$$

where F is the so called structure factor and the sum extends over all the reflections measured and their calculated counterparts respectively.The structure factor F is closely related to the intensity of the reflection it describes:

$$I_{hkl} \propto |F(hkl)|^2 $$

The R factor usually ranges between 0.6 (when comparing a random set of reflections with a given model) and 0.2 (for example for a well refined macro-molecular model at a resolution of 2.5 Angstrom). When working on very small molecules, who form extremely well ordered crystals, it is possible to attain R-factors of 0.05. Crystallographers also use the Free R-Factor and the symmetric R-Factor to describe the quality of a model.