The signal-to-noise chart is an Xbar chart.

Each plotted point, , represents the mean of the observations for subgroup, .

Term | Description |
---|---|

x_{ij} | j^{th} observation in the i^{th} subgroup |

n_{i} | number of observations in subgroup i |

The center line represents the process mean (). If you specify a historical value for an analysis that allows a historical value, then the analysis uses the historical value. Otherwise, the analysis uses the average from your data, , which is calculated as follows:

Term | Description |
---|---|

sum of all individual observations | |

total number of observations |

The value of the lower control limit for each subgroup, *i*, is calculated as follows:

The value of the upper control limit for each subgroup, *i*, is calculated as
follows:

Term | Description |
---|---|

μ | process mean |

k | parameter for Test 1 (The default value is 3 for analyses that allow custom values. The value is 3 for analyses that do not allow custom values.) |

σ | process standard deviation |

n_{i} | number of observations in subgroup i |