Understanding ASQ-CQA -Knowing The Ways
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<strong>Knowing</strong>: Process capability index<br />
Definitions<br />
• C p = Process Capability. A simple and straightforward indicator of process capability.<br />
C p = USL−LSL<br />
6σ<br />
(Estimates what the process is capable of producing if the process mean were to be centered between the specification limits. Assumes process output is approximately normally<br />
distributed.)<br />
• C p, lower = ũ−LSL<br />
3σ<br />
(Estimates process capability for specifications that consist of a lower limit only (for example, concentration). Assumes process output is approximately normally distributed.)<br />
• C p, upper = USL−ũ<br />
3σ<br />
(Estimates process capability for specifications that consist of a upper limit only (for example, strength). Assumes process output is approximately normally distributed.)<br />
• C pk = Process Capability Index. Adjustment of Cp for the effect of non-centered distribution.<br />
C pk = min[ USL−ũ<br />
, ũ−LSL<br />
3σ<br />
3σ<br />
]<br />
(Estimates what the process is capable of producing, considering that the process mean may not be centered between the specification limits. (If the process mean is not centered, Cp<br />
overestimates process capability.) Cpk
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