RE: Special Causes Indicators in Red Bead

[wmack@pec.com]

>Deming [said], "Sometimes there will be defects where the cause will never 
be known". Can we call it special or common then? Do you think maybe there 
needs to be a new operational definition--"unknown cause" 

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The definitions for "assignable/special cause" and "common cause" from Walter Shewart, I believe, only focused on whether it was probably economically viable to identify a cause.  It had nothing to do with the source of the variation, only whether it was worthwhile trying to identify a source.  

Wayne Mack
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