Center line

Line(s) on a control chart between which the statistic under consideration lies with a very high probability if the process is in-control. Action limits are typically set ±3σ from the center line assuming the plotted data is normally distributed.

Lines on a control chart together with the center line define the interval where the process statistic under consideration resides with a very high pre-set probability if the process in in-control.

Line(s) on a control chart between which the statistic under consideration lies with a high probability if the process is in-control. Warning limits are typically set ±2σ from the center line assuming the plotted data is normally distributed.

Part of a control chart, line(s) used to judge the stability of a process. On a Shewhart chart, and others, these line(s) represent control limits, or action limits. Other than on acceptance control charts, the control limits are not specification limits. The lines are statistically determined boundaries for the expected deviations from the center line due to random causes alone.

Using empirical evidence and economic considerations, the lines representing the limits are place a distance of ± z standard deviations from the center line. The lines provide a means to evaluate the stability of a process, i.e. an in-control or out-of-control process.