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 $\overline{x}\pm 2s/\sqrt{n}$, where $\overline{x}$ 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 $\overline{x}\pm 3s/\sqrt{n}$. A simulated measurement bias of one standard deviation is applied after day 10.