After a model is fitted, we get coefficients for each variable. In general, the relevant test is whether a coefficient is zero or not. If it is zero, it can be safely removed from the model. But sometimes we want to do more complex tests, involving possibly several variables, for example, testing whether the combined coefficients of variable1 and variable2 are equal to variable3.
The way this works is that we will define a contrast, and we will then estimate the significance for that contrast. We will do this using the multcomp package, which allows us to test linear hypotheses for lots of models.
