In statistical process control, what does a 'shift' in the mean of a control chart indicate?

In statistical process control, a 'shift' in the mean of a control chart indicates that there has been a sustained change in the average value of the process being monitored. A control chart is a graphical tool used to monitor a process over time, typically showing the process mean, upper control limit (UCL), and lower control limit (LCL). These limits are calculated based on the inherent variability of the process when it is operating in a stable, predictable manner. When the data points on the control chart consistently move above or below the established mean line, this signifies a shift in the process. This shift suggests that a special cause of variation has entered the process, affecting the central tendency of the data. For instance, if monitoring concrete compressive strength, a shift upward might indicate a change in cement source or an improved mixing procedure, leading to consistently higher strengths. Conversely, a shift downward could signal a problem such as incorrect batching of materials or a change in aggregate grading, resulting in lower strengths. The shift warrants investigation to identify the root cause and implement corrective actions if the shift is undesirable, or to standardize the changes if the shift represents an improvement.