What is the primary purpose of control limits in statistical quality control?

The primary purpose of control limits in statistical quality control is to detect process changes. Control limits, which are typically set at three standard deviations from the process mean, provide a framework for identifying variations in a process that are beyond what is considered normal or expected. When a process is in control, the data points will vary within these control limits; if they fall outside of these limits, it indicates that a significant change may have occurred within the process, meriting further investigation.

This ability to detect changes is critical for maintaining quality standards and responding proactively to issues that may arise in production or service delivery. By monitoring these control limits, quality control analysts can distinguish between common cause variation, which is inherent to the process, and special cause variation, which could indicate problems that need to be addressed.

In contrast, while setting operational specifications, measuring defect rates, and adjusting calibration settings are all important aspects of quality management, they do not specifically relate to the function of control limits, which are focused on monitoring and controlling the process over time.