The control chart is a graph used to track and monitor how a process develops or changes over time. A control chart always has a central line for the average, an upper line for the upper control limit and a lower line for the lower control limit. These lines are determined from historical data. These lines are determined from historical data. By comparing current data to these lines, you can draw conclusions about whether the process variation is consistent or is unpredictable. Class, when should you use a control chart? What is the basic procedure?