Estimation of anomalous signal in diffraction data.

It is difficult to judge objectively the amount and resolution limit of the anomalous signal present in diffraction data. While several criteria can be used for this purpose, the usefulness of these indicators varies and depends on factors such as the data redundancy, the accuracy of the estimation of intensities and their uncertainties, and the properties of the anomalously scattering atoms in the crystal. Various indicators give an approximate measure of the anomalous signal, but do not provide a reliable guarantee that the crystal structure will be solved. The ultimate criterion of the anomalous signal is a successful structure solution. The current data-acquisition and phasing methods are so advanced that it is often possible to phase reflections and obtain preliminary electron-density maps in a few minutes, while the crystal still resides at the experimental station and further data can be collected if necessary.