Stability Analysis Rules for Control Charts

Jay Arthur
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A stable process produces predictable results consistently. An unstable condition can either be a point, set of points or trend in your data. Stability rules and control chart 1, 2 and 3 sigma zones are used to determine unstable conditions.

The QI Macros SPC Software for Excel will turn any unstable point or condition red.

Stability Analysis Rules Used in QI Macros 2006 and previous versions:

• Do six or more points trend in the same direction?

Stability Analysis rules available in QI Macros 2007:

Rules from Juran, Montgomery and other publications are available. See our control chart rules article on how to change and customize stability analysis rules in QI Macros 2007.

If the answer was Yes to any of these questions, then the process contains unstable points or conditions:
Use cause-effect analysis to determine and resolve the root cause of the instability.

Source for stability analysis rules: Introduction to Statistical Process Control, 4th edition pp 172-175, Montgomery.

For more help, download our Choosing the Right Control Chart quick reference card.

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