What statistical parameter in SPC directly indicates process drift?

In Statistical Process Control (SPC), a shift in the mean (average) of the process is the statistical parameter that directly indicates process drift. SPC involves monitoring a process over time to identify and control variation. Control charts are used to visualize the process data and identify trends or patterns. A process drift occurs when the process mean gradually shifts away from its target value. This shift is readily apparent on a control chart as a series of consecutive points falling on one side of the center line or exhibiting a consistent upward or downward trend. While other statistical parameters, such as increased variance or the presence of outliers, can also indicate process instability, a sustained shift in the mean is the most direct indicator of process drift and the need for corrective action to bring the process back into control. Rules are used to detect shifts, such as eight points in a row above or below the center line.