The Six Sigma method measures the capability of a process using the 'sigma level'. The aim is to achieve a sigma level of at least six, which equates to less than 3.4 DPMO.
The Sigma level is calculated using the normal distribution. The idea is that the span between the upper and lower specification limits should be at least 12 standard deviations (six standard deviations on each side). Because it is not usually practical to set the processes mean exactly on target, and the mean of most processes is subject to drift, a 1.5 standard deviation offset is assumed in converting between DPMO and Sigma Level.
Other sigma levels are:
Sigma Level

Parts Per Million (PPM)

3

66811

4

6210

5

233

6

3.4

