Control Chart

Also called: statistical process control The control chart is a graph used to study how a process changes over time. Data are plotted in time order. 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. 

Upper and lower control limits (sometimes called "natural process limits") that indicate the threshold at which the process output is considered statistically 'unlikely' and are drawn typically at 3 standard deviations from the center line

By comparing current data to these lines, you can draw conclusions about whether the process variation is consistent (in control) or is unpredictable (out of control, affected by special causes of variation).

The control chart was invented by Walter A. Shewhart while working for Bell Labs in the 1920s. The company's engineers had been seeking to improve the reliability of their telephony transmission.

When to Use a Control Chart

• When controlling ongoing processes by finding and correcting problems as they occur.
• When determining whether a process is stable (in statistical control).
• When determining whether your quality improvement project should aim to prevent specific problems or to make fundamental changes to the process. 

Out-of-control signals

• A single point outside the control limits. In Figure 1, point sixteen is above the UCL (upper control limit).
• 7 successive points are on the same side of the center line.