Select the method or formula of your choice.

Each plotted point represents the proportion of defectives for one subgroup, *p*_{i}:

Data points on a P chart follow the binomial distribution.

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

x_{i} | number of defectives for subgroup i |

n_{i} | size of subgroup i |

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

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

number of defectives in subgroup | |

size of subgroup |

Each subgroup proportion 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 |
---|---|

proportion of defectives for subgroup | |

process proportion | |

size of subgroup | |

moving range of length 2 |

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

or

The UCL for each subgroup is equal to the lesser of the following:

or

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

process proportion | |

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

size of subgroup | |

see "Sigma Z" |