A control chart is a graphic display of data that illustrates the results of a process over time
The main use of control charts is to prevent defects, rather than to detect or reject them
Quality control charts allow you to determine whether a process is in control or out of control
When a process is in control, any variations in the results of the process are created by random events; processes that are in control do not need to be adjusted
When a process is out of control, variations in the results of the process are caused by non-random events; you need to identify the causes of those non-random events and adjust the process to correct or eliminate them