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 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
Zone A: n of n+1 points above/below 2 sigma
Zone B: n of n+1 points above/below 1 sigma
n points in a row above/ below center line
Trends of n points in a row increasing or decreasing
Zone C: n points in a row inside Zone C (hugging)
n points in a row alternating up and down
Zone C:n points in a row outside Zone C
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.
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 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.
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.
Why Choose QI Macros Over Other Control Chart Software?
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