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Statistical fluctuations in counting rates etc., as well as defects in the structural model, can introduce bias in the estimation of parameters by least-squares refinements. Of the residuaIs in common use, only unweighted R2 = Σ(Io - Ic)2 is free from statistical bias. Order-of-magnitude estimates of the bias can be derived, but it seems better to avoid it by adjusting the weights. To the second order, refinement of R2 is unbiased if the intensity used in calculating the usual weights is not Io but ⅓ (Io + 2Ic). There seems to be no simple method of avoiding bias in R1.
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