The Western Electric rules were codified by a specially-appointed committee of the manufacturing division of the Western Electric Company and appeared in the first edition of a 1956 handbook, that became a standard text of the field.
[2] Their purpose was to ensure that line workers and engineers interpret control charts in a uniform way.
The presence of points outside the control limits is identified as an instability pattern.
Data satisfying any of these conditions as indicated by the control chart provide the justification for investigating the process to discover whether assignable causes are present and can be removed.
The false alarm rate rises to one out of every 91.75 observations when evaluating all four rules.