Stability Analysis and Control Chart Rules

Common Control Chart Rules for Stability Analysis

Control chart limits and rules are used to detect statistically unlikely patterns.

Control chart limits and stability analysis rules

Control Chart Rules Used by Various Industries and Experts

Control chart rules can vary slightly by industry and by statistician. However, most of the basic rules used to run stability analysis are the same. Here is a list of the most common control chart rules which are all available in QI Macros SPC Software for Excel.


Control Chart Rule

West-gard Nelson- Juran AIAG Mont-gomery Western Electric IHI
n points above UCL or below LCL
1 1 1 1 1 1
Zone A: n of n+1 points above/below 2 sigma
2 2 2 2 2 2
Zone B: n of n+1 points above/below 1 sigma
4 4 4 4
n points in a row above/ below center line
8 9 7 8 8 8
Trends of n points in a row increasing or decreasing
7 6 6 6 6
Zone C: n points in a row inside Zone C (hugging)
15 15 15 15
n points in a row alternating up and down
14 14 14
Zone C:n points in a row outside Zone C
8 8 8
Zone B: n points above/ below 1 sigma; 2 points one above, one below 2 sigma

Western Electric Rules were first developed almost 100 years ago.
Western Electric Control Chart Rules


Nelson Rules are an expanded set of rules developed to cover increasingly rare conditions.
Nelsons Control Chart Rules

  • 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.
  • AIAG Rules are recommended by the Automotive Industry Action Group
  • Healthcare Rules are recommended by the Institute for Healthcare Improvement

How to Change Control Chart Rules in QI Macros


QI Macros default control chart rules for stability analysis are the Montgomery rules from Introduction to Statistical Process Control, 4th edition pp 172-175, Montgomery.

QI Macros makes it easy to change to one of the other sets of control chart rules using its Control Chart Rules sub-menu.

For more information and detailed instructions on how to customize your own control chart rules, see this article:
How to Change Control Chart Stability Rules


QI Macros Control Charts will Conduct Stability Analysis and turn any unstable point or condition red.

example of control chart after running stability analysis using control chart rules

QI Macros control charts also turn the markers for unstable points into diamonds instead of squares.

This helps to identify unstable points and trends when printing control charts on a black and white printer.

example of how unstable points are highlighted


Stability Analysis Options in QI Macros

Once you run a macro, the Control Chart Menu gives you various Control Chart Options. You can Re-Analyze Stability after adding data, change the color of unstable conditions moving in the desired direction to Green, Show Stability Rules that are violated or Clear Stability Analysis.

stability analysis options in QI Macros

Why Choose QI Macros Over Other Control Chart Software?



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