Select the method or formula of your choice.

The plotted points represent the defect rate for each subgroup, :

Data points on a U chart follow the Poisson distribution.

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

number of defects for subgroup | |

size of subgroup |

The center line represents the process mean, . If you do not specify a historical value, then Minitab uses the mean from your data, , to estimate .

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

number of defects in subgroup | |

size of subgroup |

Each subgroup rate is converted to a z-score ():

Next, a moving range of length 2 is used to evaluate the variation in the z-scores and calculate Sigma Z ():

where 1.128 is an unbiasing constant.

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

defect rate for subgroup | |

process mean | |

size of subgroup | |

moving range of length 2 |

The LCL for each subgroup is equal to the greater of the following:

or

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

process mean | |

parameter for Test 1. The default is 3. (For more information go to Select tests for special causes for Laney U' Chart.) | |

size of subgroup | |

see "Sigma Z" |