Capability indices: Cpk
Cpk is a capability index that tells how well a system can meet specification limits . Cpk calculations use estimated sigma and, therefore, shows the system's "potential" to meet specifications. Since it takes the location of the process average into account, the process does not need to be centered on the target value for this index to be useful.
If Cpk is 1.0, the system is producing 99.73% of its output within specifications. The larger the Cpk, the less variation you will find between the process output and specifications.
If Cpk is between 0 and 1.0, not all process output meets specifications.
If the system is centered on its target value, Cpk should be used in conjunction with the Cp index. Cpk and Cp will be equal when the process is centered on its target value. If they are not equal, the smaller the difference between these indices, the more centered the process is.