Stability Analysis and Performance Chart Rules

Common Performance Chart Rules for Stability Analysis

A stable process produces predictable results consistently. It 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 causes. An unstable (i.e., out of control) condition can either be a point, set of points or trend in your data.

Performance Chart rules and Performance Chart limits are used to detect statistically unlikely patterns in the data.

For example, one point outside of the UCL or LCL should only happen 3 times in 1000 points; so if you only have 20 data points, it's statistically unlikely, but not impossible.

Performance Chart limits used to perform stability analysis

 

Performance Chart Rules Used by Various Industries and Experts

Performance 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 Performance Chart rules.
All of these options are available in the ChartSmartXL SPC Software for Excel.

#

Performance 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.
  • 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 Performance Chart Rules in the ChartSmartXL

how to change Performance Chart rules used for stability analysis in ChartSmartXL

ChartSmartXL default Performance Chart rules for stability analysis are the Montgomery rules from Introduction to Statistical Process Control, 4th edition pp 172-175, Montgomery.

ChartSmartXL makes it easy to change to one of the other sets of Performance Chart rules using its Performance Chart Rules sub-menu.

 


 

ChartSmartXL Performance Charts will Conduct Stability Analysis and turn any unstable point or condition red.

example of Performance Chart after running stability analysis using Performance Chart rules

ChartSmartXL Performance Charts also turn the markers for unstable points into diamonds instead of squares.

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

Performance Chart rules with unstable points highlighted by marker

 

Stability Analysis Options in ChartSmartXL

Once you run a macro, the Performance Chart Menu gives you various Performance 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 ChartSmartXL

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