upper control limit

The upper control limit is a key component of statistical process control, a method used to monitor and maintain the quality of ongoing processes, such as manufacturing. Control charts, which are graphical displays, are central to this method. They help determine if variations in a process's output are due to random, common causes or identifiable, assignable causes.

The upper control limit, along with a lower control limit, forms the boundaries on a control chart. These limits are typically set at three standard deviations above and below the process mean. If a sample's data falls within these limits, the process is considered in control. However, if a data point falls outside the upper control limit, it signals an out-of-control condition, indicating that corrective action is needed to restore the process to acceptable quality levels.12