More Patterns of Instability

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:

1. Cycles

2. Freaks

3. Gradual Change in Level

4. Grouping or Bunching

5. Instability

6. Interaction

7. Mixtures

8. Stable Forms of Mixture

9. Unstable Forms of Mixture.

10. Stratification

11. Sudden Shift in Level

12. Systematic Variables

13. Tendency of One Chart to Follow Another

14. 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.

While Western Electric, AIAG, Nelson, Boeing, and GE Six Sigma “Rules” may look different at first, they all have the same derivation.