(Note: The hat over the sigma symbol indicates that this is an estimate of standard deviation, not the true population standard deviation.)īecause control limits are calculated from process data, they are independent of customer expectations or specification limits.Ĭontrol rules take advantage of the normal curve in which 68.26 percent of all data is within plus or minus one standard deviation from the average, 95.44 percent of all data is within plus or minus two standard deviations from the average, and 99.73 percent of data will be within plus or minus three standard deviations from the average. Mathematically, the calculation of control limits looks like:
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