Definition of Upper Control Limit (UCL):
Upper Control Limit (note, different from USL): representing a 3 x sigma upwards deviation from the mean value of a variable (see also LCL). For normally distributed output, 99.7% should fall between UCL and LCL.
When used on control charts, the “3sigma” level can be calculated from sample-to-sample values or batch-to-batch averages using a “magic number”, and is used to flag-up unexpected deviations.
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