(1) Process Capability index also known as Cpk is a measurement of process capability shown as a ratio of the distance between the specification limits and the process mean to the process variation. To have a capability of at least 1, a process must be in statistical control and on target.
(2) The value of the tolerance specified for the characteristic divided by the process capability. Cpk and cp are common types of indices.
The Process Capability Index is a measurable property of a process to the specification, expressed as in the terms of Cpk or Cpm or as a process performance index (e.g., Ppk or Ppm). The output of this measurement is usually illustrated by a histogram and calculations that predict how many parts will be produced out of specification (OOS).