Control Charts

A Control Chart is a graph that is very popular in business and industry. These graphs are used to control variation in product production. Another use of control charts is to determine whether a quality improvement project should aim to prevent specific problems or to make fundamental changes to the process.

A control chart is a graph used to study how a process changes over time. Data are plotted in time order. A control chart 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 by adding and subtracting the standard deviation (σ) to the center. By comparing current data to these lines, you can draw conclusions about whether the process variation is consistent or unpredictable. If the process is consistent, it is called in control. If the process is unpredictable, it is considered out of control and is affected by special causes of variation. Control charts are an easy method to check the centering of the distribution and the range, or width of the distribution. If your data represents target practice; the center, or average is where the shots are clustering, and the range is how tightly they cluster.