Alternative Control Chart Constructions
In this treatment of the subject, description has been of charts with both warning and action limits, and these limits calculated with multiples of 1.96σ and 3.09σ, respectively. Some practitioners including Walter Shewhart, have argued that the coefficients imply an inappropriate degree of accuracy, and recommend that they be simplified to 2σ and 3σ. Further, in many applications the warning limits are considered to be superfluous.