Sigma Level

In addition to the conventional process capability indices such us Cp, Cpk, Pp, Ppk, process capability can also be expressed with the sigma level.

The sigma level Z describes how many standard deviations can be placed between the mean value and the tolerance limit of a process.

If you want to use short-term data to make statements about long-term capability, you can use the following formula as a projection:

The value 1.5 was determined empirically from manufacturing processes and considers that more influences affect the process in the long term than over a shorter period of time. However, the 1.5 sigma difference is not a constant, but can vary depending on the short-term value. The difference between (extreme) short-term and long-term observation is 1.5 on average.