Learn · updated 2026-10-21
The 8 Nelson rules for control charts
A control chart shows a special cause when a point falls outside the limits, but some changes show up first as patterns inside them. Lloyd Nelson's eight tests, published in 1984, are the standard list of those patterns.
The zones: the centre line ± 1σ, ± 2σ and ± 3σ, where σ is the within-process standard deviation the chart's limits are built from.
| Rule | Pattern | Usually means |
|---|---|---|
| 1 | One point beyond 3 sigma | A sudden large shift, a one-off error, a mis-measurement. |
| 2 | Nine points in a row on the same side of the center line | The mean has shifted: new material lot, setting, operator or gauge. |
| 3 | Six points in a row steadily increasing or decreasing | Drift: tool wear, temperature, a depleting chemical. |
| 4 | Fourteen points in a row alternating up and down | Over-adjustment, or two alternating sources (two machines, two shifts). |
| 5 | Two of three points beyond 2 sigma on the same side | A medium shift, shown sooner than rule 1 would. |
| 6 | Four of five points beyond 1 sigma on the same side | A small, sustained shift. |
| 7 | Fifteen points in a row within 1 sigma of the center line | Variation is smaller than the limits assume: limits computed from old data, or mixed streams averaging out. |
| 8 | Eight points in a row beyond 1 sigma on either side | Points avoid the centre: often two processes mixed together. |
Don't switch them all on
Each rule adds a chance of a false alarm on a perfectly stable process. With all eight on, a stable process trips a rule far more often than with rule 1 alone, and people learn to ignore the chart. LimitLine turns on rules 1, 2, 3, 5 and 6 by default, a common choice that catches shifts and drifts without crying wolf; rules 4, 7 and 8 are one click away when you are hunting a specific problem such as over-adjustment or mixed streams.
What to do when a rule fires
- Note which point and which rule. The rule tells you what kind of change to look for: a jump, a shift, a drift.
- Go to the process around that time: material, settings, people, gauges, environment.
- Write the cause next to the point (point notes, in LimitLine Pro), fix it, and once it is fixed leave those points out of the limits.
Don't adjust the process just because a rule fired. Find the cause first; adjusting a stable process makes it worse.
Nelson or Western Electric?
The Western Electric rules (1956) are the older set; Nelson's list builds on them with the same zone idea. Rules 1, 5 and 6 match Western Electric's first three, while Nelson's 2 uses nine points in a row on one side instead of eight. If your company standard names one set, follow it.
Sources
- L. S. Nelson (1984), “The Shewhart Control Chart—Tests for Special Causes”, Journal of Quality Technology 16(4), 237–239
- Western Electric Company (1956), Statistical Quality Control Handbook
- NIST/SEMATECH e-Handbook of Statistical Methods, §6.3.2 What are Variables Control Charts?