Value defining an intended level of stability for a process.
Notes: - A control limit may be a statistical value or be related to a predetermined target value, calculated, for example, from a target measurement uncertainty.
- A typical control chart will consist of a centre line that reflects the level around which the plotted value can be expected to vary. In addition, this control chart will have two lines, called control limits, placed one on each side of the centre line, which define a band within which the value can be expected to lie randomly for a process in a state of statistical control.
- Control limits on a Shewhart control chart are placed at a distance of ±2σ and ±3σ where σ is the known or estimated standard deviation of the population. This gives approximate probabilities of 0.05 and 0.003, respectively, of finding values outside the control limits. These limits are known as the upper and lower warning limit (UWL and LWL) and upper and lower action limit (UAL and LAL), respectively. See Shewart means chart - Fig. 1.
- A typical response to a value outside a warning limit is to monitor the process and, if this condition is repeated, to stop and investigate.
- A value outside an action limit is normally taken as evidence that the process is no longer in statistical control. A typical response to such a value is to stop and investigate. (See process in a state of statistical control).
Source:
PAC, 2021, 93, 997. 'Metrological and quality concepts in analytical chemistry (IUPAC Recommendations 2021)' on page 1023 (https://doi.org/10.1515/pac-2019-0819)