Improved R-factors for diffraction data analysis in macromolecular crystallography

The quantity R(sym) (also called R(merge)) is almost universally used for describing X-ray diffraction data quality. Here, we prove that R(sym) is seriously flawed, because it has an implicit dependence on the redundancy of the data. A corrected R-factor, R(meas), is introduced as the equivalent robust indicator of data consistency. In addition, we introduce R(mrgd), an R-factor that reflects the gain in accuracy upon averaging of equivalent reflections, as a useful indicator of the quality of reduced data. These new data quality indicators better reveal the benefits of highly redundant data and should stimulate improvements in data quality through increased merging of data from multiple crystals.