Additive and interactive terms

Additive model

What model adds each predictor's contribution independently?

Interaction model

What model lets one predictor's effect depend on another?

Parallel lines

What do additive models with a categorical predictor produce?

Different slopes

What can an interaction model produce for different groups?

Interaction term

What extra term models how one effect changes with another variable?

Multiplication

What operation creates an interaction term?

Slope difference

What does an interaction coefficient with a categorical predictor capture?

Same

In an additive model, how does a predictor's effect compare at different values of the other predictors?

Different

In an interaction model, how does a predictor's effect compare at different values of the other predictors?

y ~ a * b

Which formula includes both predictors and their interaction?

y ~ a + b

Which formula includes two predictors without an interaction?

a:b

What formula term specifies only the interaction?

Standalone effect

What does the coefficient of a predictor in an interaction model describe when the other predictor is 0?