Adjusted Control Limits for U Charts

Abstract: U charts are used for count data following the Poisson distribution.  However, the U chart has symmetrical control limits when the Poisson distribution is nonsymmetrical.  As a result, the upper control limit can have a rate of false detection as high as 1 in 11.5 points plotted.  This can result in wasted resources investigating false signals.  Further, the lower control limit has a rate of false detection consistently above 1 in 1000.  This makes it slow to detect improvements in quality.  Adjusted control limits are provided that correct both these problems.  The adjusted control limits result in a U chart truly based on the assumption of the Poisson distribution.

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