The control chart is a graphical comparison of process performance that is used to study how a process might vary over time. The control chart usually will include predetermined control limits which are shown as the UCL (upper control limit) and the LCL (lower control limit). The control chart also shows a "mean" or average line which is based on historical data. The control charts primary purpose is to show a comparison of the current data to the historical data that is plotted on the chart. Doing this helps the user see whether the variation in a process is consistent (controlled) or the process has random variation (out of control, also referred to as special causes of variation).
When should you use a control chart?
Using a Control Chart:
1. Select the appropriate control chart: variables - X chart, R Chart, σ chart Attributes - p chart, np chart, c chart.
2. Establish the time period that you will be collecting data for.
3. Collect data
4. Analyze data looking for "out of control" signals.
5. Work to find the root cause of any "out of control" signals.