Shewhart Control Charts

Menu: QC.Expert Control charts Control charts are constructed to decide whether a process is under statistical control and to monitor any departures from this state. This means that stability of some process properties over time is tested using certain statistical assumptions about the process (data it produces). Commonly considered properties are mean, variance (standard deviation), distribution shape or proportion of nonconforming items. The Shewhart control charts were invented in 1932. They are based on monitoring events, which are very unlikely when the controlled process is stable. Any incidence of such an event is taken as an alarm signal suggesting that stability of the process was broken and the process changed. Upon receiving such signal, possible causes of the change should be investigated and some correcting steps taken. One example of such an unlikely event is the situation when the control limits (UCL or LCL) are exceeded. They are constructed as ±3 limits, so that when the process is under control, they are exceeded with relative frequency of 0.27%. In addition to the LCL and UCL limits, warning limits (LWL and UWL) are constructed at ±2 as well as limits at ±. More complicated rules specifying event having low probability when the process is under control can be constructed. Some of these rules can be selected in the Shewhart control chart dialog panel, by default all available rules are selected. They are: