Quality Advisor

A free online reference for statistical process control, process capability analysis, measurement systems analysis, control chart interpretation, and other quality metrics.

Capability indices

Capability indices use estimated sigma .


cp formula

Cp for one-sided specifications

If you are using one sided specifications, use the following formulas to determine the Cp:

Upper specification:

cpu formula

Lower specification:

cpl  formula


cr formula


cpk formula


Zmin is the smaller of Zupper and Zlower.

Using estimated sigma:

z upper formulacpm formula z lower formula


Where: sigma for cpm

T = specification target (nominal)

Xi = a given individual reading of " i "

n = total number of individual readings

= symbol for summation

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