Which statement is not a signal of an out-of-control process on a control chart?

• A. Two of three consecutive data points are above the upper control limit or below the lower control limit.
• B. Four of five consecutive data points are between one and two standard deviations above or below the mean.
• C. Four consecutive data points are increasing or decreasing.
• D. Nine consecutive data points are on one side of the mean.

Answer: (A)

Control charts look for nonrandom patterns that indicate the process is out of control. Signals come from consistent shifts or patterns relative to the center line and the sigma bands, not from a couple of points just crossing the limits in a small window. The well-known rules include: a single point outside the control limits; two of three consecutive points beyond two standard deviations on the same side of the mean; four of five consecutive points beyond one standard deviation on the same side; nine consecutive points on one side of the mean; and four consecutive points that are all increasing or all decreasing. The statement about two of three consecutive points being above the upper control limit or below the lower control limit doesn’t align with these rules. It relies on the control limits themselves in a short window, which can occur by chance and is not considered a standard signal of an out-of-control process. The other statements describe recognized patterns: consistent movement beyond a sigma boundary, a clear trend, or a long run on one side of the mean. If you want, we can walk through a small example to see how each pattern appears in data.