A formula is a list of identifiers and operators defining the model terms to be analysed. For example:

Variety + Nitrogen | Main effects of Variety and Nitrogen |

Variety . Nitrogen | Interaction term for Nitrogen and Variety |

Variety * Nitrogen | Same as Variety + Nitrogen + Variety.Nitrogen |

The following operators are commonly used in model formulae:

+ | Addition | A+B+C | Main effects of A, B, and C |

. | Interaction | A.B | Interaction of A and B |

* | Cross-product | A*B | Is equivalent to A+B+A.B |

/ | Nesting | A/B | Is equivalent to A+A.B |

The following operators can also be used in formulae.

– | Subtraction |

-/ | Nested deletion |

-* | Crossed deletion |

// | Linkage of pseudo terms |