Example: A Shewhart means chart with mean values from a process in a state of statistical control with a simulated change in the process mean after 10 days is shown in Fig. 1.
Note: Nordtest terms a control chart with measured quantity values scaled by the mean or target value an "X-chart". ISO uses the term "X-control chart" for a control chart which plots single values.
Figure 1: Shewhart means chart of the means of duplicate analyses of a quality control material, twice per day (d) over 20 days. Each point is the mean of the day’s four results (\(n = 4\)). Upper and lower warning limits (UWL and LWL) are at \(\bar{x} \pm 2s/\sqrt{n}\), where \(\bar{x}\) is the process mean, \(s\) is the standard deviation under repeatability conditions of measurement, and upper and lower action limits (UAL and LAL) are at \(\bar{x} \pm 3s/\sqrt{n}\). A simulated measurement bias of one standard deviation is applied after day 10.
PAC, 2021, 93, 997. 'Metrological and quality concepts in analytical chemistry (IUPAC Recommendations 2021)' on page 1040 (https://doi.org/10.1515/pac-2019-0819)