Which variation refers to the inherent variation of a process in statistical control?

The inherent variation of a process that remains constant over time while the process is in focus is known as Common Cause Variation. This type of variation is considered a natural part of the process and results from the normal functioning of the system. It encompasses the minor fluctuations that occur due to factors that are always present in the system, such as material differences, environmental conditions, and operator actions, among others.

When a process is statistically controlled, these common cause variations indicate that the process is stable and predictable, allowing for the establishment of performance standards. Quality control efforts typically focus on identifying and reducing special cause variations, which are unexpected and may indicate a problem or an anomaly in the process. By distinguishing between these types of variations, practitioners can better understand process variability and make decisions on process improvements.