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 
