Free R value: a novel statistical quantity for assessing the accuracy of crystal structures

THE determination of macromolecular structure by crystallography involves fitting atomic models to the observed diffraction data1. The traditional measure of the quality of this fit, and presumably the accuracy of the model, is theR value. Despite stereochemical restraints2, it is possible to overfit or 'misfit' the diffraction data: an incorrect model can be refined to fairly good R values as several recent examples have shown3. Here I propose...