It is known that even when a process is in control (that is, no special causes are present in the system, (p = p0.), there is approximately a 0.0027 probability that a point exceeding 3-sigma control limits. Even in a properly constructed control chart, it may be possible that a special cause is detected without necessarily having happened. For a control chart using 3-sigma limits, this false alarm occurs on average once in every 1/0.0027 or 370.4 observations. Therefore, the in-control average run length (ARL0) of a chart is 370.4. In a less restrictive condition, as happens with high quality processes, it is more realistic to consider a false alarm probability slightly greater, for example, of 0.005, that is, a false alarm takes place on average once in every 1/0.005 or 200 observations.