What does it mean if a system shows a single statistic outside of control limits?

When a system shows a single statistic outside of control limits, it indicates that the process is experiencing variation that is not due to common causes. Control limits are established based on the inherent variability of the process when it is stable; a data point falling outside these limits suggests that an unusual event or variation has occurred. This can signal that the system is out of control, meaning that there may be assignable causes influencing the process that needs to be investigated and addressed to return it to a state of control. The presence of a data point outside the control limits is a prompt for further analysis and corrective actions to mitigate any issues that may be affecting the quality or consistency of the output.