It can be referred to as Zusl and Zlsl and Zoverall.

]]>1. “In a stable process, the mean naturally shifts as much as 1.5 sigma in the long term on either side of its short-term value.” This statement is self-contradictory. A stable process doesn’t shift—by definition. If a stable process could shift that much, then control charts or other tools for assessing stability would be useless as they would need to differentiate between a “natural” shift and “unnatural” shifts. For example, if there is one or more points beyond control charts limits that could come from a shift of the mean less than 1.5 sigma, why does that make the process unstable rather than stable within its natural shift?

2. Six sigma and other process improvement methodologies use facts to make better decisions. There is no factual basis for the 1.5 sigma shift. (If you think there is then provide it.)

3. Sigma merely represents either the percent meeting specifications or the percent not meeting specifications (e.g., DPMO). Hence, there is no need to make the calculations so complicated. Percentages are easily understood while the sigma scale is not—hence, the reason for your article.

4. The calculations you show assume a normal distribution. But since sigma is merely percent meeting specifications, then normality is not required for determining a sigma value. Just determine the percent meeting specs using whatever distributional assumption (e.g., exponential for cycle time) and convert to the sigma scale value. But since you are going to explain the sigma scale as percent meeting spec, the conversion is completely unnecessary. ]]>