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Hi there,
I'm writing an online course on Quality Assurance for purchasing professionals. I have some prior background in SPC but I'm finding some advice that's contradictory to what I learned and I want to be sure I can explain different schools of thought...and even better, say which one is right when they contradict. This site has been great at explaining these details.
This issue concerns calculating control limits for X-bar charts. It's pretty clear that they should be calculated using factor "A2" from published tables and that ASQ in their "control chart template" calls them "3 sigma" limits. (Template is at http://asq.org/learn-about-quality/data-collection-analysis-tools/overview/control-chart.html).
Now I'm trying to explain the Western Electric Zone concept which requires knowing not just three sigma limits but one and two sigma limits to see if the process is under control. The explanations I've seen seem to advocate using the sigma-bar value and multiplying it by 1,2, or 3.
Going back to the ASQ template, it calculates 1,2 and 3 sigma control limits that are not proportional to each other. The 1 sigma upper limit is much more than a third of the three sigma upper limit.
This is a convoluted way of getting around to asking how ASQ and others calculate those lower sigma limits. I haven't been able to get into their spreadsheet and look or find an explanation on line.
Thanks much
Dick Locke
I'm writing an online course on Quality Assurance for purchasing professionals. I have some prior background in SPC but I'm finding some advice that's contradictory to what I learned and I want to be sure I can explain different schools of thought...and even better, say which one is right when they contradict. This site has been great at explaining these details.
This issue concerns calculating control limits for X-bar charts. It's pretty clear that they should be calculated using factor "A2" from published tables and that ASQ in their "control chart template" calls them "3 sigma" limits. (Template is at http://asq.org/learn-about-quality/data-collection-analysis-tools/overview/control-chart.html).
Now I'm trying to explain the Western Electric Zone concept which requires knowing not just three sigma limits but one and two sigma limits to see if the process is under control. The explanations I've seen seem to advocate using the sigma-bar value and multiplying it by 1,2, or 3.
Going back to the ASQ template, it calculates 1,2 and 3 sigma control limits that are not proportional to each other. The 1 sigma upper limit is much more than a third of the three sigma upper limit.
This is a convoluted way of getting around to asking how ASQ and others calculate those lower sigma limits. I haven't been able to get into their spreadsheet and look or find an explanation on line.
Thanks much
Dick Locke