Rearrangement of Cruickshank's formulae for the diffraction-component precision index

The formulae for the diffraction-component precision index introduced by Cruickshank [(1999), Acta Cryst. D55, 583-601] are simplified using two approximations. A rearranged formula for the precision index is presented which can readily be calculated from experimental data. It is shown that the precision varies as (resolution)^5/2 if R and completeness are maintained. It varies as (completeness of intensity measurement)^-5/6 and its dependence on the inclusion of solvent atoms is discussed.