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Statistical Techniques and 6 Sigma Capability Stuff

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Author  Topic: Capability Stuff 
Marc Smith Cheech Wizard Posts: 4119 
posted 23 November 1999 12:09 PM
Subject: Inspection Criteria RE1 Date: Wed, 15 Sep 1999 09:20:54 0400 (EDT) From: Greg Gogates iso25@fasor.com To: iso25@quality.org Date: Tue, 14 Sep 1999 23:04:58 +0200 Dear Group, SNIP I believe that the 33% limit is obtained by assuming the upper and lower spec limit to be 3 sigma. If we relate this back to SPC practically: Specification: 100 ± 1 the ± 1 is assumed a three sigma value and the 100 as the mean, now......... ZONE A (Stable zone) would be the mean ± 1 sigma thus 100 ± 0.333 ZONE B (Warning zone) would be the mean ± 2 sigma thus ± 100 ± 0.667 ZONE C (Action zone) would be the mean ± 3 sigma thus 100 ± 1 This theory then also traces back to the Process capability indices Cp = USL  USL / 6 sigma NOTE: to manufacture within specifications the distance between the USL (upper specification limit) and the LSL (lower specification limit) must be less than the width of the base of the normal distribution thus the distance between 3 sigma and + 3 sigma) If Cp is < 1, the process is not capable Cp = 1, the process is marginally capable and Capability index I trust that this helps and makes some sense. Regards in Quality IP: Logged 
Don Winton Forum Contributor Posts: 498 
posted 23 November 1999 09:10 PM
Good Post. I agree. Regards, IP: Logged 
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