when a chart is first set up, the control limits are not that critical. (Now, _that_ is a heretical statement!) Previously, no one recognized the variations for what they were, so every wild point on the brand new chart leads quickly to understanding of causes and corrections (prevention, in ISO9000 -speak). A Run Chart is all it takes at this point. After things settle down some, a control limit will give guidance on the value of any system changes/ improvements that may reduce the overall variation (i.e., average, if we're discussing flaws).
sorry if my response misled anyone - I recall the question regarded the calculation, not the logic of improvement.
Jay
"Simon, Steve, PhD" wrote:
Jay Warner writes:
>The 'party line' is to take the first 30 or so points, calculate
>limits, throw out any outside ones & add more at the end, until you
>have 30 points, all of which are inside the control limits.Actually, I have heard the opposite. Out-of-control points do not have THAT much of an impact on the control limits, so that recomputing control limits after removing out of control points is not a good use of your time. Wheeler makes the suggestion that the time spent fine tuning the chart might be better spent investigating why the original points are out of control.
I am not an SPC guru, though, so take my advice with a grain of salt.
Steve Simon, [EMAIL PROTECTED], Standard Disclaimer.
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Jay Warner
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