## There are many statistically valid patterns of instability. Every pattern of instability is based on the properties of the normal curve and the 68-95-99.7 Rule.

- Most are based on the Western Electric Handbook first published in 1954.
- Different organizations use slightly different variations of these patterns to interpret control charts.
- Your industry or company may decide to modify these guidelines to suit your unique manufacturing situation.

### There are 4 tests of instability that many people know of as the Western Electric Rules. In addition to these tests, the Western Electric Handbook also identifies “14 Other Unnatural Patterns of Variation.”

- All are based on the probabilities and properties of the normal curve.

### The 4 common Western Electric tests of instability are:

- Point outside the control limits.
- 2 of 3 points between 2s & 3s from the mean.
- 4 of 5 points between 1s & 3s from the mean.
- 8 points in a row on one side of the centerline.

### The 14 Other (Western Electric) Unnatural Patterns of Variation are:

- Cycles
- Freaks
- Gradual Change in Level
- Grouping or Bunching
- Instability
- Interaction
- Mixtures
- Stable Forms of Mixture
- Unstable Forms of Mixture.
- Stratification
- Sudden Shift in Level
- Systematic Variables
- Tendency of One Chart to Follow Another
- Trends

### The AIAG Rules for interpreting patterns of instability are:

- 1 point outside of control limits.
- Run of 7 points on one side of the mean.
- Trend up or down of 7 points in a row.
- Recurring cycles.
- Other non-random patterns.

### The Nelson Rules were developed in the 1980s by Dr. Lloyd Nelson. Dr. Nelson put numbers to some of the Western Electric Other Unnatural Patterns. Nelson Rules are:

- 1 point outside of control limits.
- 2 of 3 points in zone A (between 2s and 3s from the mean).
- 4 of 5 in zones A or B (between 1s and 3s from the mean).
- Run of 9 on one side of the mean.
- 15 in a row near the centerline.
- Trend of 6 points in a row increasing or decreasing.
- 8 in a row not within 1s of the mean (on both sides of the mean).
- 14 points in a row that alternate up and down.

### Boeing’s AQS (Advanced Quality System) Rules for interpreting control charts are:

- 1 point outside of control limits.
- 2 of 3 points in zone A (between 2s and 3s from the mean).
- 4 of 5 in zones A or B (between 1s and 3s from the mean).
- Run of 8 on one side of the mean.
- Lumping.
- Mixtures.
- Trend.
- Recurring

cycles. - Strays.
- Process shifts.
- Few points within limits.
- Too few discrete levels.

### The General Electric Six Sigma Rules for interpreting control charts are:

- 1 point outside of control limits.
- 2 of 3 points in zone A (between 2s and 3s from the mean).
- 4 of 5 in zones A or B (between 1s and 3s from the mean).
- Run of 8 on one side of the mean.
- Trend.