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- Index
Cp
Cp is a capability index that is used by Statistical Process Control (SPC) to measure the ability of the variation to fit within the specification limits. It is calculated as follows:
where LSL is the lower spec limit, USL is the upper spec limit and S is an estimate of the standard deviation. The larger Cp is, the better the fit. Cp is required to evaluate the protection and to select variables sampling plans for defective units with standard deviation known and 2 specification limits.