The lower and upper control limits, LCL, and UCL, respectively, are computed using the following formulas:
P chart LCL =
P chart UCL =
NP chart LCL =
NP chart UCL =
where:
is the average proportion of nonconforming items taken across subgroups
ni is the number of items in the ith subgroup
K is the sigma multiplier and is set to 3 by default