Process Capability Index - Measure of the Inherent Capability
This index provides a measure of the inherent capability of a process by simply relating the 6σ spread of the process to the tolerance band. The assumption is made that the mean m of the process lies at mid- tolerance, as shown in the figure below. If the Cp index is less than 1, then a significant proportion of the process output will be out with specification, even when the process mean is centred within the tolerance band.
If the Cp index value is equal to 1, then, provided the process mean is centred within the tolerance band, only 3 parts per thousand (0.3 per cent) will be outwith the specification. If the Cp index value is greater than 1, and the process mean is centred, then a correspondingly higher proportion of the output will conform to specification. For example, GE/Motorola's six-sigma (ie ±6 sigma) programme equates to a Cp index value of 2, and a non- conformance rate of 3.4 parts per million.