Control limits are statistical values that define the acceptable range of variation for a process. They are used in process monitoring and quality control to distinguish between common cause variation, which is inherent in the system and expected, and special cause variation, which is due to identifiable and unusual events that can have a significant impact on the process.

Control limits are calculated from a set of data assumed to be in control, which means only common causes of variation are present. The most common type of control limits are the upper control limit (UCL) and the lower control limit (LCL). These limits are derived from the process data itself, often using the process mean and standard deviation.

In statistical process control (SPC), control limits are used to determine if a process is in a state of statistical control. If the process data points fall within the control limits, it is assumed that the process is stable and only common causes of variation are at play. If data points fall outside the control limits, it suggests that special causes of variation are present, indicating a potential problem that needs to be investigated.

It's important to note that control limits do not represent the specification limits or tolerances of a product or process. They are simply a tool to detect changes in the process that could indicate a loss of control.

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Control limits, also known as natural process limits, are horizontal lines drawn on a statistical process control chart, usually at a distance of ��3 standard deviations of the plotted statistic from the statistic's mean.read more >>