The mean (average) of the run length. This is the average number of samples that are taken until an alarm is signaled by the control chart.
It is a funny thing about life: if you refuse to accept anything but the best you very often get it.
— W. Somerset Maugham
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The purpose of using a control chart is to detect a shift in the process mean, of a specific size. To detect a shift of two standard-deviations-of-the mean, enter the value 2.