Chemical Crystallography: when are ‘ bad data’ ‘good data’?

Abstract There is an increasing trend to reduce diffraction data collection to a set of numbers and the crystallography to a set of criteria. Many of these criteria have been passed on through generations of crystallographers. In Chemical Crystallography, the majority of small-molecule singe crystal diffraction data are still collected using laboratory equipment. Herein we review the methods for analysing data, discuss some of the influences on data quality and try to determine whether some of the validation tropes widely used are sound advice, or ‘Chinese Whispers’.