Testing for Interaction in Multiple Regression

Contrary to a recent claim, the inclusion of a product term in a multiple regression is a legitimate way to test for interaction. The unstandardized coefficient and the t-test for the product term are unaffected by the addition of arbitrary constants to the variables in the model. Certain other statistics are affected by this change, however, indicating that some hypotheses relating to interaction are not meaningfully testable unless variables are measured on ratio scales.