The run chart shown previously provides a clear, graphical representation of the amount of variation in a process, and is certainly more informative than the simple table of values. However, control charts go a lot further in that they provide a mechanism to signal the presence of a special cause of variation affecting the process.
When a chart is first prepared for use with a particular process, 3 lines called control limits or control lines are added to the graph where the values will be plotted. These lines are:
These control limits will have been calculated from data already collected from the process. For a completely new process, the chart will have to be used without limits until a suitable number of samples have been collected (usually about 25).
A point which falls above the UCL or below the LCL signals the presence of a special cause and indicates that the process is not in a state of statistical control.
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