B. Toby stated that "the most important way to determine the quality of a Rietveld fit is by viewing the observed and calculated patterns graphically and to ensure that the model is chemically plausible." 46 What role does this 'human intuition' in assessing the quality of a fit play, and how can this be replicated by ML models? To begin, we can consider why goodness of fit statistics alone are insufficient. All goodness of fit is not equal: in a Rietveld refinement, completely unfitted diffraction features may increase the χ 2 by the same amount as slightly incorrect peak shapes, but normally the former are a much more serious concern for the analyst, as they represent at best a missing component of the sample phase composition, or at worst show that the entire model is wrong, for example, that the real material has a different symmetry to the one modelled.