Graphical tools used in statistical and quality control of a process to represent the state of control. They are used to indicate whether changes are required to be made to control parameters, as well as identify the parameters that require control. They are also used to predict the control of the process. A process that is stable but lies outside desired control limits requires understanding of the causes for control. They are also known as process-behaviour charts or Shewhart charts after American engineer Walter A. Shewhart (1891–1967) who devised them in the 1920s.