Process Capability Index (Cpk) is a statistical metrics that consider the variation of the process. It is widely used to grade the performance of the process to meet its specifications.


Cpk is a standard index to state the capability of one process, the higher the Cpk value the better the process is. 

 

For instance, ‘Machine A’ has a Cpk of 1.7 and ‘Machine B’ has a Cpk of 1.1. From the Cpk value, one can derive that ‘Machine A’ is better than ‘Machine B’.


A Cpk value less than 1.0 is considered poor and the process is not capable.


A value between 1.0 and 1.33 is considered barely capable, and a value greater than 1.33 is considered capable.


But, you should aim for a Cpk value of 2.00 or higher where possible.

 

A Cpk of 2.00 implies that the process produces only ‘2 defective unit’ out of 1 BILLION Units!


In the Six Sigma quality methodology, process performance is reported to the organization as a sigma level. The higher the sigma level, the better the process is performing.


Read also: Difference between Process Capability (Cpk) and Process Performance (Ppk)