Publication date: 05/24/2021

Control Limits for P and NP Charts

The lower and upper control limits, LCL, and UCL, respectively, are computed using the following formulas:

P chart LCL = Equation shown here

P chart UCL = Equation shown here

NP chart LCL = Equation shown here

NP chart UCL = Equation shown here

where:

Equation shown here is the average proportion of nonconforming items taken across subgroups

Equation shown here

ni is the number of items in the ith subgroup

K is the sigma multiplier and is set to 3 by default

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