Stability Analysis Rules

Stability Analysis Rules

 

Stability rules identify and highlight points or trends that need to be investigated. They vary by industry.

 

Common Control Chart Rules for Stability Analysis

 

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

 

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 Rules for Control Charts

 

WECO Rules - the Original Four Rules

 

Western Electric Rules are the original set of four control chart rules. Western Electric Rules can be used with all Excel control charts and are sometimes referred to as WECO Rules.

 

Western Electric Rules are Available in QI Macros

 

List of the Original Four Western Electric Rules

1. One point above UCL or below LCL
2. Two points above/below 2 sigma
3. Three out of four points above/below 1 sigma
4. Eight points in a row above/below the center line

These were expanded by Nelsons Rules in the 1950s.

 

Western Electric Rule 1: One point above UCL or below LCL

 

Western Electric Rule 2: Two points above/below 2 sigma

 

Western Electric Rule 3: Three out of four points above/below 1 sigma

 

Western Electric Rule 4: Eight points in a row above/below the center line

 

How to Change to Western Electric Rules in QI Macros

 

QI Macros use Montgomery's Rules as the default. However, you can easily switch to the Western Electric Rules using the QI Macros menu.

 

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

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

 

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

 

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.

 

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.