This chapter follows on from the principles of the previous chapter. The challenge is the intent and attempt to define the accuracy of a result, a data set or a fitted structure, compared with the estimation of variance or precision in the previous chapter. A key issue considered by many X-ray absorption spectroscopy researchers is that of `XAFS accuracy', meaning the ability of an X-ray absorption fine-structure data point or set to determine a parameter or structure, yet it is and can be more insightful than a simple definition of variance, precision or reproducibility. One of the key issues is the understanding, calibration or determination of some class of specific systematic errors or uncertainties in the data set, and how they may be assessed, corrected for or included in the estimation of data-point accuracy and hence absolute measurement, fitting parameters and structural uncertainty.