177 The Six Sigma Zone

The objective of having a process operate with a capability index of 1.5 to 2.0 is a reasonable and economic objective. However, the arguments commonly used to explain the economics of this objective can only be described as statistical snake oil—a blend of tortured computations and incompatible, highly questionable assumptions, having a hypnotic effect and often resulting in a suspension of critical thinking. Therefore, since the conclusion is right, but the argument used to support the conclusion is complete nonsense, this paper begins with a theoretically sound and rigorous justification for the economics of operating with capabilities in the 1.5 to 2.0 range. Furthermore, this justification allows you to convert capabilities into the one metric that all managers understand: the excess cost of production. This justification is more rigorous, more comprehensible, and more useful than the common arguments involving parts-per-million nonconforming. Finally, in the interests of exorcism, this paper will conclude with an explanation of the fallacies contained in the common parts-per-million argument.