Contrast with interaction terms

[alanocallaghan]

From John Sorkin via email

Does the package automatically handle an interaction term, e.g. for the model and contrast statement below,

fit1 <- lm(y~ x + z + x*z)

contrast(x=10, z=10)

Will the contrast statement include in its calculation the x*z interaction? Some testing I have done suggests it does not.

If I am correct (that the contrast statement written above does NOT include the interaction term in its computation), is there a way to specify the interaction in the contrast statement?

I have tried contrast(x=10, z=10, xz=100), contrast(x=10, z=10, xz=100), contrast(x=10, z=10, “xz”=100) none of which run; all produce an error message.

In a model with an offset, will the contrast statement work and include the offset in its computation, for example

fit2 <- lm(y~ x + z + x*z + offset=offset)

contrast(fit2, x=10, y=10)

Some experimentation suggests the contrast function does not account for the offset.

Assuming I am correct, that the offset term is NOT honored in the contrast statement, is there a way to write a contrast statement that will work?