Stability Analysis Rules for Control Charts

Jay Arthur
888-468-1537
303-756-9144
KnowWare International, Inc.
DBA LifeStar
2253 S. Oneida
Ste 3D
Denver, CO 80224

We work with companies
that want to fire up their profits using
Lean Six Sigma

Copyright © 2011

A stable process produces predictable results consistently. A stable process is in statistical control and shows variation due to common causes. An unstable process is out of statistical control and shows variation due to special cause. An unstable (i.e., out of control) 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.

The QI Macros also turns the markers for unstable points into diamonds instead of squares so that unstable points and trends can still be identified when printing control charts on a black and white printer.

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 to the current version:

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.

Westgard Rules are used with Levey Jennings Charts in laboratories. They are a slightly different subset of traditional rules with a couple of special rules.

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.

Download the FREE 30-day Evaluation copy of the QI Macros Excel SPC Software for Six Sigma

Download
QI Macros
30-Day Trial Now!

Or Buy It Now!