If staying in the spec is critical, then the preferred value is 1.67, meaning the closest spec is 5 standard deviations away. Finally, if the value of Cpk is 2, then the closest spec is 6 standard deviations. This is the definition of Six Sigma, and you can then claim that you have...
Six-Sigma-Material.com A Cpk of 2.0 corresponds to approximately 6 sigma (short term) performance or 4.5 sigma (long term) if applying the shift. In other words, the sigma score (z) is the Cpk * 3. The Z in the above formula refers to a short term sigma, short term Z. ...
Let's visualize the meaning of the centering and spread so that its relevance gets crystal clear. Alow spreadalone is not enough as we see in the lower-left corner of the diagram below. Although our process is very stable, we have many, manyoff-specs. ...