What is the statistically determined line on a control chart that marks the boundary for upper or lower limits of variation?

• A. Lower specification limit
• B. Mean line
• C. Standard deviation line
• D. Control limit

Answer: (D)

In a control chart, you chart the process mean as the center line and set boundaries called control limits that define the range of normal, common-cause variation. These control limits are calculated from the process data (typically set at about ±3 standard deviations from the mean). They act as the statistically determined boundary for variation: data points outside these limits suggest the process may be out of control due to special causes and warrant investigation. The center line simply marks the average performance, not the boundary of acceptable variation. Specification limits, set by design or customer requirements, are separate and indicate acceptable product tolerances, not the process's inherent variability. So the boundary line described is the control limit.