Understanding ASQ-CQA -Knowing The Ways
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Recommended values<br />
Process capability indices are constructed to express more desirable capability with increasingly higher values.<br />
Values near or below zero indicate processes operating off target (ũ far from T) or with high variation.<br />
(<strong>The</strong> target process mean is T, the estimated mean of the process is ũ and the estimated variability of the process (expressed as a standard deviation) σ. )<br />
Fixing values for minimum "acceptable" process capability targets is a matter of personal opinion, and what<br />
consensus exists varies by industry, facility, and the process under consideration. For example, in the<br />
automotive industry, the Automotive Industry Action Group sets forth guidelines in the Production Part Approval<br />
Process, 4th edition for recommended Cpk minimum values C pk = min[ USL−ũ<br />
, ũ−LSL<br />
] for critical-to-quality CTQ<br />
3σ 3σ<br />
process characteristics. However, these criteria are debatable and several processes may not be evaluated for<br />
capability just because they have not properly been assessed.<br />
Since the process capability is a function of the specification, the Process Capability Index is only as good as<br />
the specification. For instance, if the specification came from an engineering guideline without considering the<br />
function and criticality of the part, a discussion around process capability is useless, and would have more<br />
benefits if focused on what are the real risks of having a part borderline out of specification. <strong>The</strong> loss function of<br />
Taguchi better illustrates this concept.<br />
At least one academic expert recommends the following:<br />
(cont. ………)<br />
Charlie Chong/ Fion Zhang