Re: Use of Control Charts

[DANSWART@aol.com]

Steven_S_Prevette@rl.gov writes:

<< I do use sigma to estimate variation in the graphs I do, but that does 
have the danger of an outlier "blowing up" the standard deviation. >>
--------------------

Steve,

In your discussion, does "Sigma" =  "Standard Deviation?"  I have come to 
think of Sigma as the statistic you get when correctly constructing a control 
chart (ala Shewhart, Deming, Wheeler) where order of observation is a 
critical concept, and Standard Deviation as the statistic from distribution 
theory which, I guess, is a purely mathematical construct which presupposes 
an idealized universe.  I also understood that one should not substitute the 
Standard Deviation (as found on handheld calculators and Excel functions) in 
place of the Sigma properly calculated for control charts.  Or are they 
interchangeable terms?

Dan
Dan Swart, CPA, APC
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