Nelson rules

Nelson rules are a method in process control of determining if some measured variable is out of control (unpredictable versus consistent). Rules, for detecting 'out-of-control' or non-random conditions were first postulated by Walter A. Shewhart in the 1920s. The Nelson rules were first published in the October 1984 issue of the Journal of Quality Technology in an article by Lloyd S Nelson.Note that the rule is concerned with directionality only. The position of the mean and the size of the standard deviation have no bearing.The side of the mean for the third point is unspecified.The side of the mean for the fifth point is unspecified.