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.