What are Control Limits?

Control limits are the horizontal lines in a control chart that delineate the upper and lower limits of the acceptable range of results for a process. When plotted data exceeds a control limit, it indicates that a process is out of control, and requires management attention. Control limits are set at three standard deviations on either side of the mean.

Control Limits vs. Specification Limits

Specification limits are results mandated by customer requirements. These requirements may be looser or tighter than the control limits for a process. This means that a process may be in control but not meeting specifications, or (the reverse) the process is out of control while still meeting a customer’s specification limits.