What is the primary purpose of a control limit?

The primary purpose of a control limit is to establish a threshold that helps in assessing the variability of a process and determining whether it remains in a state of statistical control. Control limits are used in quality control charts to distinguish between common cause variation, which is inherent to the process, and special cause variation, which indicates that something unusual has occurred.

Control limits serve as a benchmark for measuring process performance and understanding the variation within that process. When a process operates within these limits, it indicates that the process is stable and consistent. If the data points fall outside these control limits, it signals that an investigation is necessary to identify and rectify any underlying issues.

While identifying quality defects, managing project timelines, and estimating cost reductions are important aspects of project and quality management, they do not encapsulate the primary function of control limits. Control limits specifically relate to analysis of process stability and variation, which is essential for maintaining quality standards and improving processes.