Run Length Variability and Three Sigma Control Limits

Abstract This article provides an argument against the use of three sigma limits for control charts. An alternative proposal to use control limits narrower than three sigma control limits is made. The main reason for narrowing the control limits is to achieve a smaller variability in the frequency of process resetting as well as false alarms. An example is given illustrating why a good statistical design of control charts needs to avoid the use of three sigma control limits.