Six Sigma vs. Ppm
The following shows how the Six Sigma Index compares with Process Performance and the Ppm Index.
Assume the true mean randomly shifts with a standard deviation of 0.5σ then
Assuming the process is centered within the specifications (Ppm will equal Pp). Ppm can be calculated using the long-term sigma, 1.118σ, as follows. Our spec range, per the above diagram is 12σ.
A Ppm of 1.79 could be considered equivalent to Six Sigma.
The following table shows an equivalent Ppm for various values of the Six Sigma Index.