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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 in the data.

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
1	n points above UCL or below LCL	1	1	1	1	1	1
2	Zone A: n of n+1 points above/below 2 sigma	2	2	2	2	2	2
3	Zone B: n of n+1 points above/below 1 sigma		4	4	4	4
4	n points in a row above/ below center line	8	9	7	8	8	8
5	Trends of n points in a row increasing or decreasing	7	6	6	6		6
6	Zone C: n points in a row inside Zone C (hugging)		15	15	15		15
7	n points in a row alternating up and down		14	14	14
8	Zone C:n points in a row outside Zone C		8	8	8
9	Zone B: n points above/ below 1 sigma; 2 points one above, one below 2 sigma	4

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.