I just looked at the poll regarding ISO registration. My production manager agrees it is losing favor and my head salesman also agrees. We are in the electronics assemblies business. Are their responses typical in this industry? What is the opinion in other industries?
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atul_khandekar 4/11/2003
Check out this article on my website:
http://www.symphonytech.com/articles/processcapability.htm
firebrew 2/11/2003
It sounds like you are using your 30 piece sample to construct a frequency distribution, and from that calculating a standrd deviation with which to estimate capability. That being the case, you must use Ppk to estimate capability. By definition, Cpk requires evidence of statistical stability, which is best determined by using a Shewhart type control chart. This requires taking mesurements over time and determining that there aren't any signs of non-randomness (assignable causes) in your process.
Ppk is not hamstrung by that requirement, but is also not as reliable a predictor of future performance. Ppk merely quantifies present and past perfomance rather than predicting futuer performance.
John Bruman
QA Manager