Struggling to Understand Control Chart Rules?

QI Macros add-in for Excel will create control charts in seconds
and analyze stability so that you don't have to!

a process must be stable before you can analyze process capability to determine if it meets customer specifications.
a process must be stable before beginning an improvement project.

Control limits (±1, 2, 3 sigma) are calculated from the data.
Zones represent the spaces between the limits.
Control chart rules are then applied to data points as they move through those zones.
Unstable points and trends are identified for investigation.

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

Control chart rules can vary slightly by industry. However, most of the basic rules used to run stability analysis are the same.

QI Macros uses the rules from Introduction to Statistical Process Control, 4th edition pp 172-175, Montgomery as its default. QI Macros also offers these other rules and makes it easy to change from one rule set to another using its Control Chart Rules sub-menu.

#	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 Healthcare and Westgard Rules state that 2 out of 3 points can be both Above AND Below centerline All other rules state that 2 out of 3 points MUST be above OR below the centerline	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: 4 points above/ below 1 sigma; 2 points one above, one below 2 sigma	4

To view the Control Chart Rules that are currently enabled, select "Show Current Rules" from the Control Charts (SPC) > Control Charts Rules menu:

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

Nelson Rules expanded the set of rules 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. They are only slightly different from Nelson/Juran Rules, using 9 instead of 8 for detecting runs.
Healthcare Rules are recommended by the Institute for Healthcare Improvement. They only use rules 1, 2, 4, 5, and 6.