On the significance of 'non-significant' reflexions

The information content of so-called 'unobserved', or 'non-significant' reflexions obtained by diffractometer measurements has been experimentally studied, using data and input parameters for three previously deter- mined crystal structures. It can be concluded that even those reflexions having the largest σ(I)/I values contain significant information about both thermal and positional atomic parameters. It has been possible to carry out an acceptable refinement of one structure on the basis of only the set of structure factors discarded by the original investigator. Inclusion of the weakest reflexions in the refinements lowers the standard deviations, and so should be standard practice.