4. Our advice

4.1 Stay calm in the face of alarms

When drawing a control chart, we generally set the control limit at 3 standard deviations, which represents 99.73% of the population, subject to the normality of the parent population. This means that 0.27% (risk α) of the population falls outside this limit. This percentage corresponds to 1 in 370 observations. This reflects the possibility of observing a non-acceptable value even though it is part of the acceptable population. If such a case arises, it is essential to confirm this "anomaly" by additional measurements (confirmation or invalidation). This is referred to as the α-risk, i.e. the risk of a false alarm.