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 C_p 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 C_p 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 C_p 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 C_p index value of 2, and a non- conformance rate of 3.4 parts per million.