Control Chart Rule Violations
The following are some of the more common rules that may be used to indicate "Out-of-Control" conditions or process changes.
The software tests for each of these rule violations, except for Cyclical Effects. In addition, an Excessive Moving Range and Zone Rules may also tested as well as zone rules.
The Moving Range is calculated by the absolute value of the difference between each two consecutive points. If Moving Ranges were plotted, an Excessive Moving Range would be found above the upper control limit on the Moving Range Chart and signifies a change may have taken place.
The following codes are used in display of control test violations.
- O: Point outside the limits
- R: Run (8 points above or below the centerline)
- T: Trend (6 points increasing/decreasing)
- L: Lack of Variability (15 within one standard error)
- E: Excess Variability (8 outside one standard errors)
- A: Alternating Values (14 points alternating up/down)
- M: Excessive Moving Range (Outside limits on MR chart)
- 1: Zone 1 Rule (4 out of 5 outside one standard error)
- 2: Zone 2 Rule (2 out of 3 outside two standard errors)