The plotted points represent the defect rate for each subgroup, :
Data points on a U chart follow the Poisson distribution.
The center line represents the process mean, . If you do not specify a historical value, then Minitab uses the mean from your data, , to estimate .
Each subgroup rate is converted to a z-score ():
Next, a moving range of length 2 is used to evaluate the variation in the z-scores and calculate Sigma Z ():
where 1.128 is an unbiasing constant.
The LCL for each subgroup is equal to the greater of the following: