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They are a set of modified Western Electric rules, developed by James Westgard and provided in his books and seminars on quality control. [1] 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).
An example of a Levey–Jennings chart with upper and lower limits of one and two times the standard deviation. A Levey–Jennings chart is a graph that quality control data is plotted on to give a visual indication whether a laboratory test is working well. The distance from the mean is measured in standard deviations.
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Analyse-it Quality Control & Improvement edition provides the standard Analyse-it statistical analyses above, plus procedures for statistical process control, including Shewhart, Levey-Jennings, CUSUM, and EWMA control charts, process capability analysis, and pareto analysis.
Control charts are graphical plots used in production control to determine whether quality and manufacturing processes are being controlled under stable conditions. (ISO 7870-1) [1] The hourly status is arranged on the graph, and the occurrence of abnormalities is judged based on the presence of data that differs from the conventional trend or deviates from the control limit line.
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The Levey-Jennings chart differs from the Shewhart Individuals control chart in the way sigma is estimated. The Levey-Jennings chart uses the long-term (i.e., population) estimate of sigma whereas the Shewhart chart uses the short-term (i.e., within the rational subgroup) estimate. John 72.153.169.233 21:41, 30 November 2009 (UTC)