One measurement exceeds 3 standard deviations either above or below the mean of the reference range.Ģ consecutive measurements exceed 2 standard deviations of the reference range, and on the same side of the mean. One measurement exceeds 2 standard deviations either above or below the mean of the reference range. They are plotted on Levey–Jennings charts, wherein the X-axis shows each individual sample, and the Y-axis shows how much each one differs from the mean in terms of standard deviation (SD). They are a set of modified Western Electric rules, developed by James Westgard and provided in his books and seminars on quality control. They are used for laboratory quality control, in "runs" consisting of measurements of multiple samples. The Westgard rules are a set of statistical patterns, each being unlikely to occur by random variability, thereby raising a suspicion of faulty accuracy or precision of the measurement system.
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