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A statistical process control (SPC) chart is a valuable tool used in quality control to monitor and analyze the variation in a process over time. Its primary function is to distinguish between two types of variation: random (common cause) variation and variation that is attributed to specific, identifiable causes (assignable cause variation).

When using an SPC chart, the control limits provide a framework for understanding the natural variation within the process. Points that fall within these control limits indicate that the variability is due to common causes, which are typically stable and inherent to the process. Conversely, points that fall outside of these limits suggest that there may be assignable causes influencing the process, warranting further investigation.

This ability to differentiate helps teams focus on improvements where they are most needed and prioritize actions that can lead to significant enhancements in quality and efficiency. Thus, the key function of a SPC chart is to clearly differentiate between random variation and variation with assignable causes, supporting effective decision-making and process improvement initiatives.

The other choices, while related to aspects of quality control, do not capture the primary discriminative power of SPC charts as accurately as the ability to delineate between random variation and assignable causes.