In crystallography, the R-factor (sometimes called residual factor or reliability factor or the R-value or RWork) is a measure of the disagreement between the crystallographic model and the experimental X-ray diffraction data - lower the R value lower is the disagreement or better is the agreement.
[2] It is defined by the following equation: where F is the so-called structure factor and the sum extends over all the reflections of X-rays measured and their calculated counterparts respectively.
Model inadequacies such as incorrect or missing parts and unmodeled disorder are the other main contributors to
, making it useful to assess the progress and final result of a crystallographic model refinement.
1000 atoms) usually form better-ordered crystals than large molecules, and thus it is possible to attain lower R-factors.
is computed according to the same formula given above, but on a small, random sample of data that are set aside for the purpose and never included in the refinement.
, but the two statistics should be similar because a correct model should predict all the data with uniform accuracy.
are similarly used to describe the internal agreement of measurements in a crystallographic data set.