Effective resolution of macromolecular X-ray diffraction data

'Effective resolution' is a practical quantitative measure of the extent, quality, and completeness of X-ray diffraction data. Sparrow's [Astrophys. J. (1916), 44, 76-86] criterion for the separation of just resolved peaks is applied to a typical electron density peak shape, which, in turn, is the Fourier transform of an average form factor derived from the observed Fhkl's. Numerical studies of a Gaussian form factor, with appropriate modifications for atomic scattering factors, give a quantitative picture of the dependence of resolution on both the amount of data and the underlying structural features. The relation between effective resolution and nominal resolution (drain) depends only on the ratio of the limiting high-resolution peak width to dmin.