With ideas of what defines a new material in mind, we can now consider the evidence necessary to determine if one has been produced. The creation and testing of hypotheses is a fundamental feature of science. The best strategy for testing a hypothesis can depend on the context; a positive testing strategy is one that looks only for evidence that confirms the hypothesis. It can be appropriate in some special situations where only one working hypothesis exists, but in general it is undesirable, and inappropriate adoption of this strategy is a well-known cognitive bias. Given the large number of materials now known, any hypothesis about discovery of a new material cannot be tested solely by confirmation, but must be also tested against falsification -i.e. tests should be carried out to determine if the sample under investigation is instead a known material. Any known material that might realistically form under the synthesis conditions should be considered as a candidate for such testing. For example, if a synthesis is carried out using three elements, X, Y and Z, with the intention to form the ternary compound XYZ, it is prudent to assess whether the diffraction pattern (or any other analysis) can instead be explained by known compounds that can be formed by the reactants, e.g. the binary compounds XY, XZ and YZ, or other ternary compounds, like XY 2 Z 4 . Likewise, the unreacted starting materials should also be eliminated from enquiry. If known materials can adequately explain the experimental evidence, then there is no need to conclude that new materials have been formed. This is a statement of Occam's Razor, which was also expressed in similar terms by Russell: "Whenever possible, substitute constructions out of known entities for inferences to unknown entities." A positive testing strategy, one that only looks for confirmation of the hypothesis of the presence of a new material, and does not look for alternative explanations involving known materials, is inadequate in a field as well established and densely populated as that of materials chemistry. Instead, any report of a new material must be accompanied by an explanation as to why the experimental evidence is better explained by a new material, compared to one or more known materials. In some instances, powder x-ray diffraction might not be capable of providing evidence that can differentiate two materials. Even under perfect experimental conditions, there is information loss by the nature of the PXRD experiment, and this means that there is no one-to-one correspondence between PXRD pattern and structure, so that many theoretical structures may give identical diffraction patterns. Therefore, even if an excellent match between model and experimental XRD can be achieved, this still does not guarantee the modelled compound is the correct one. Schlesinger et al. point out that the "...mere existence of a plausible crystal structure, a good Rietveld fit with a smooth difference plot, acceptable R-values and a successful checkCIF test does not justify the attribute 'correct structure'."