RE: Cause source - system v. employee

[Steven_S_Prevette@rl.gov]

Wayne - I agree with your second paragraph, but not necessarily the first
paragraph.  You still look for the causes of common cause variation - you
just have to be sure to look at the entirety of the data rather than just an
individual result.  

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My understanding of control charts is that the only time that a cause for a
variation can be objectively identified is when a special cause has
occurred.  The criteria used is timing, what happened just prior to the
change in the data?

I see root cause analysis as something that can be applied to either common
or special cause variation.  In the case of common cause variation, however,
the result of the analysis is a hypothesis and further experimentation is
required to make an objective determination.  This is done by correcting the
suspected cause and verifying that a special cause variation occurs on the
control chart immediately following the change.

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Steve Prevette
Site Technical Authority for Statistical Trending
Environment, Safety and Health
Fluor Hanford, A Fluor Global Services Company
ASQ Certified Quality Engineer
steven_s_prevette@rl.gov
509-373-9371