# Process Capability Index (CPK)

The process capability index (Cpk), a statistical tool that measures the ability of a process, to produce output within specified limits for customers, is called a statistical tool. It measures the producer’s ability to produce a product within the customer’s tolerance range. Cpk can be used to determine how close you are to a target and how consistent your performance is relative to that target. Cpk will give you the best-case scenario for your current process. Cpk can also predict the future performance of the process, provided that performance remains consistent over time.

- The process is operating on the edge of the Cp value equals the process capability index (Cpk). The Six Sigma standard design parameters are met and the production capacity is sufficient.
- If Cpk is lower than zero, the process means has broken one of the specification boundaries.
- Cpk greater than zero, but less than one means that the process mean is within the specifications, but some of the manufacturing output has gone beyond them.
- If Cpk is greater than one, the process mean is correctly centered.

The formula for Process Capability Index can be used with possibly bilateral tolerances (where LSL and USL have been provided). If there is only one specification (unilateral), then the value for Ppk and Cpk are calculated using the limit of the specified specification.

Process performance indices Pp and Ppk use the sample standard deviation (s) formula to estimate sigma. Other methods are available to estimate the total process sigma.

Process Capability Index (Cpk) and Ppk are used to center the process between the midpoints of the specifications. This performance index might not be optimal if the customer chooses another point to serve as the target. Cpm is calculated by adding a target value.

### Related articles