Control Limits

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:

• the Upper Control Limit (UCL)
• the Centre Line which represents the process mean
• the Lower Control Limit (LCL)

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.

Question

The graph below shows the same data as the run chart, except that the control limits have been calculated and added. Looking at the graph, would you say that this process is performing as well as it could?

Yes
No

Choose an option and then click the button to check your answer.