The Psychology of Quality and More

Process Capability Index

A common Process Capability measure, Cp (often called a Process Capability Index), indicates how well the process distribution fits within its specification limits, and is simply the ratio of the specification width to the variation width. 

The problem with Cp is that it does not take account of how well the process distribution is centered within its limits, which can result in a process with both a low Cp and many rejects. The solution to this is a second measure, Cpk, which measures a similar ratio, but considers only the variation half that is closest to the specification limits. Thus Cp and Cpk, taken together, give a measure of both the potential and centering of the process distribution within the specification limits.

See also:

Toolbook chapter: Process Capability