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 689599.7 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.”
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 nonrandom 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.

