Publisher Summary This chapter focuses on the free R value and other applications of cross-
validation in crystallography. Powerful methods have been developed to lower the chances
of misinterpreting or overinterpreting diffraction data. Some of the more important methods
measure the agreement of the structure with empirical rules about protein folds,
comprehensive conformational analyses, the real-space correlation coefficient, and the free
R value. The protein-folding rules and conformational analyses depend on empirical …