Publication date: 05/24/2021

Control Limits for U Charts and C Charts

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

U chart LCL = Equation shown here

U chart UCL = Equation shown here

C chart LCL = Equation shown here

C chart UCL = Equation shown here

The limits vary with ni.

ui is the number of nonconformities per unit in the ith subgroup. In general, ui = ci/ni.

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

ci is the total number of nonconformities in the ith subgroup

ni is the number of inspection units in the ith subgroup

Equation shown here is the average number of nonconformities per unit taken across subgroups. The quantity Equation shown here is computed as a weighted average

Equation shown here

N is the number of subgroups

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