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 |