An interaction effect occurs when the effect of one explanatory variable on a response variable depends on the value of another explanatory variable. For two factors with
levels 0 and 1 and cell means 
, the two-factor interaction contrast is
 |
It is zero when the two factor effects are additive. In a regression model, an interaction is commonly represented by a product term, for example
 |
where 
measures the interaction on the scale of the model.
