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 P_{pm} for various values of the Six Sigma Index.

ppm | 6σ Index |
---|---|

0.89 | 3 |

1.19 | 4 |

1.49 | 5 |

1.79 | 6 |

2.09 | 7 |

2.39 | 8 |

2.68 | 9 |

2.98 | 10 |