Control Charts

Control charts are one of the seven basic quality tools and are essential for monitoring process stability in the Control Quality process. A control chart has a center line representing the process mean, an upper control limit (UCL), and a lower control limit (LCL). These limits are typically set at three standard deviations (3 sigma) from the mean. Data points that fall within the control limits indicate the process is stable and in control.\n\nA process is considered out of control when data points fall outside the control limits or when non-random patterns appear in the data, such as the Rule of Seven (seven consecutive points on one side of the mean). When a process is out of control, it requires investigation and corrective action. It is important to distinguish between control limits (which represent the natural variation of the process) and specification limits (which represent the customer's quality requirements).\n\nControl charts help the project team distinguish between common cause variation (inherent to the process) and special cause variation (caused by unusual circumstances). Common cause variation is normal and expected; special cause variation signals a problem that needs attention. This distinction is critical for making appropriate quality management decisions.