Control charts are used to track regular measurements of an ongoing process, and to signal when such a process had reached the point of going "out of control" (i.e. may no longer be governed by the same properties, such as mean or standard deviation). The control chart has upper and lower control limits for sample statistics provided by successive samples taken over time. If sample statistics meet certain criteria for being "extreme", (e.g. too far away from the mean), this is a signal to investigate the process and determine whether anything is wrong.