control chart

"A graphical method for evaluating whether a process is or is not in a 'state of statistical control.' The determinations are made through comparison of the values of some statistical measure(s) for an ordered series of samples, or subgroups, with control limits" [ASQ]. In healthcare laboratories, the Levey-Jennings chart is commonly used to plot the result observed for a stable control material versus time, usually the day or run number.