To give a more specific example, if the hypothesis is that a material has ordered cations, then the experimental diffraction pattern should be considered against candidate models with both ordered and disordered cations. In the perovskite structure, the A and B cation sites are distinguished, and usually occupied by large, low charge ions on the A site, and small, higher charge ions on the B site. Differences in size and charge mean that A and B cations seldom mix to any appreciable extent, and rarely would it be necessary to test a model of say, the perovskite SrTiO 3 with Sr(II) and Ti(IV) disordered. But if two cation types are present on the B site, with similar size and charge, these may well mix, or they may order, and in this case consideration of both ordered and disordered models becomes essential. A well known example from the literature is Sr 2 FeMoO 6 , where the Fe and Mo B site cations can show different degrees of disorder. Any diffraction peaks that arise from the ordered but not disordered structure, or any peaks that change intensity appreciably between the two models, are clearly of prime importance. The absence of an expected ordering peak, even if that peak is small compared with the other diffraction features, and its absence perhaps makes only a small difference to the numerical goodness of fit, can be fatal for the hypothesis of cation ordering. The answer to the entire research question may depend on the presence or absence of relatively small diffraction features. In other situations, a peak of exactly the same size, that belongs to a minor impurity phase, may have almost no relevance to the overall research question, beyond suggesting that a small adjustment of the synthetic procedure is needed.