What is a typical signal indicating an out-of-control condition in a control chart per Western Electric rules?

• A. A single point exactly on the center line.
• B. A point outside the control limits or a run of consecutive points on the same side of the center line.
• C. A sequence of alternating up and down points within control limits.
• D. A small nonconforming sample.

Answer: (B)

Western Electric rules look for nonrandom patterns in a control chart beyond a single point outside the limits. The most typical signals of an out-of-control condition are either a point that falls outside the control limits or a run of consecutive points all on the same side of the center line. A point outside the limits suggests the process has shifted or a special cause is present, beyond what random variation would produce. Similarly, a sustained run on one side of the center line indicates the process mean may have moved, creating a persistent bias that the chart flags as out of control. In contrast, a point exactly on the center line, or a pattern of alternating up and down points still within limits, is generally consistent with common cause variation. A small nonconforming sample relates to product quality more than the statistical signals WE rules look for in chart patterns.