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An empirical relationship of use in prediction and evaluation is established between the standard uncertainty of the Flack parameter and the Bijvoet intensity ratio. The expected value of this ratio may be calculated from the chemical composition of the compound and the X-ray wavelength. Structure analyses published with intensity data have allowed various properties of the Bijvoet intensity ratio to be studied. It is found that, although the Bijvoet intensity ratio has a strong dependence on sin θ/λ, extrapolation to sin θ/λ = 0 of model intensity pairs leads to values satisfactorily close to those expected. Moreover, it is shown that there is no symmetry enhancement for general reflections of the Bijvoet ratio in agreement with theory. The behaviour of some special reflections is examined. Two methods of correcting the observed Bijvoet ratio for systematic and random effects have been tested and found to be unsatisfactory. Evidence is produced to show that standard uncertainties provided with intensities are unrealistic and that measurement protocols need improvement.

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Plots of the magnitude of D versus sin theta/lambda and the magnitude of D versus A, and other supplementary data

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