Global indicators of X-ray data quality

Global indicators of the quality of diffraction data are presented and discussed, and are evaluated in terms of their performance with respect to various tasks. Based on the results obtained, it is suggested that some of the conventional indicators still in use in the crystallographic community should be abandoned, such as the nominal resolution d_min or the merging R factor R_merge, and replaced by more objective and more meaningful numbers, such as the effective optical resolution d_eff,opt and the redundancy-independent merging R factor R_r.i.m.. Furthermore, it is recommended that the precision-indicating merging R factor R_p.i.m. should be reported with every diffraction data set published, because it describes the precision of the averaged measurements, which are the quantities normally used in crystallography as observables.