Process Capability indices

The basic Process Capability indices commonly used are Cp, Cpk, Cpm

Cp = VOC/ VOP = Tolerance/Natural Process variation = USL - LSL/ 6 std. dev.

• Cp values greater than or equal to 1 indicates the process is technically capable.
• Cp value equal to 2 is said to represent 6 sigma performance.

However, Cp index is limited in its use since it does not address centering of a process relative to the specification limits. For that reason Cpk was developed.

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Cpk = minimum (Cp lower , Cp upper)

• Cp lower = mean - LSL / 3 std dev.
• Cp upper = USL - mean / 3 std dev.
• Cpk determines the proximity of process mean to the nearest Specification limit.
• Also, at-least one specification limit must be stated in order to compute Cpk value.
• Cpk values of 1.33 or 1.67 are commonly set as goals since they provide some room for the process to drift in relation to the nominal setting

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Cpm, also known as process capability index of mean.

It is an index that accounts for the location of the process average relative to target value and is defined as.

where, T = target value (typically center of Tolerance)

• When the process average and target values are equal, the Cpm = Cpk.
• When the process average drifts from the target value, the Cpm is less than Cpk.