Method for selecting half datasets used in computing Rsplit, CC1/2 and CCstar. The following enumerated values are used: by_observation: unmerged reflection intensities are randomly divided into two half-sets of nearly equal size. As recommended in Karplus PA, Diederichs K. Linking crystallographic model and data quality. Science. 2012;336(6084):1030-1033. by_lattice: often used in serial crystallography, crystals are pre-sorted into two half datasets of nearly equal size (such as by odd vs. even crystal number).