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short communications
A quality indicator Nσ is defined that exhibits immediately the significance level of the result of a least-squares refinement. For a refinement with v degrees of freedom, Nσ = (M − ν)/(2ν)1/2 expresses the deviation of the deviance M from its expected value v in terms of its standard deviation (2ν)1/2. The deviance M is the sum of weighted squared differences between observed and model data. For any ν > 20, |Nσ| should be as small as possible; however, it should be < 3 for a sufficiently good refinement. Thus, Nσ is a quantitative measure for the conformity of data, their weights and the model, independent of the number of the degrees of freedom.