Refers to the control rule that is commonly used with a Levey-Jennings chart when the control limits are set as the mean plus/minus 2s. Any QC result outside ☓s violates this rule A run is rejected when a single control measurement exceeds the mean plus 3s or the mean minus 3s control limit. This rule identifies unacceptable random error or possibly the beginning of a large systematic error. If no relationship can be found and no source of error can be identified, it must be assumed that a single control value outside the ☒s limits is an acceptable random error. The relationship between this value and other control results within the current and previous analytical runs must be examined. This rule merely warns that random error or systematic error may be present in the test system. Remember that in the absence of added analytical error, about 4.5% of all quality control results will fall between the 2s and 3s limits. This is a warning rule that is violated when a single control observation is outside the ☒s limits. Thus 13s represents a control rule that is violated when one control observation exceeds the ☓s control limits. Most of the quality control rules can be expressed as NL where N represents the number of control observations to be evaluated and L represents the statistical limit for evaluating the control observations. Westgard devised a shorthand notation for expressing quality control rules. These rules are used individually or in combination to evaluate the quality of analytical runs. The elements of the Westgard system are based on principles of statistical process control used in industry nationwide since the 1950s.10 There are six basic rules in the Westgard scheme. James Westgard of the University of Wisconsin published an article on laboratory quality control that set the basis for evaluating analytical run quality for medical laboratories.
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