Methods that apply a function across a levels of one or more factors. It works like aggregate but returns a table instead. It also has a useNA options that adds NA as a level before applying the function.

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`formula` |
Formula defining the variables. On the left is the variable we are applying the function to, on the right, variables defining levels of the tables |

`data` |
Data.frame containing the variables |

`FUN` |
The function to apply to each subset of data |

`...` |
extra parameters to FUN |

`subset` |
Vectors defining a subset of data.frame
(see |

`na.action` |
Action functions to deal with NA in data file |

`useNA` |
Make NA a level of the factors (if any) |

xf returns an object "xf" that behaves like a table with all associated methods.

Charles-Édouard Giguère

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