Control Limits (ASTM E456). A set of boundaries on a control chart that are used as criteria for signalling the need for action or judging whether a set of data does or does not indicate a state of statistical control. If all values are within the control chart control limits this indicates a state of statistical process control. Normally three standard deviations above and below the control chart centre line, equivalent to a level of confidence of 99%. Only 1 result in 100 is expected to exceed the limit.