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. 2008;49(2):157–169. doi: 10.1093/ilar.49.2.157

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Levey-Jennings charts showing the mean value of a single control result (ordinate) versus the assay run number (abscissa). (A) shows increased dispersion (random error) in control results after assay run number 10, and (B) shows a bias or trend in the control data after assay run number 13 (systematic error).