More Out of Control Signals

Section 3 Control Charts

Lesson S03-08

Text: Section 3 pages 46 – 52

Duration: 32 minutes

X̄ Specific Out of Control Rules

X̄ specific signals of strong evidence of a special cause shifting the process mean.

• 4 out of 5 points in top or bottom third of the chart
• 2 out of 3 consecutive points in the top or bottom sixth of the chart
• 15 points in a row in the middle third of the chart
• 8 point in a row outside the middle third of the chart

The above signals apply to X̄ charts but not range charts. More generally, they depend on the statistic being plotted following a normal distribution so any charts of statistics or values that follow a normal distribution may utilize these rules.

Zone Rule Violation 4 out of 5 in Outer Third

Zone Rule Violation 2 out of 3 in Outer Sixth

Zone Rule Violation of 15 in the Middle Third

Zone Rule Violation 8 Outside the Middle Third

Minitab examples of Out of Control Examples

Minitab Rule Settings and Customization