ABSTRACT
This chapter seeks to clarify the concepts of moderation, mediation, and common cause. Moderation is another term for interaction, and is used to describe the situation where the magnitude of the effect of one variable on another depends on a third variable. The use of the language “it depends” is a hint that we are likely talking about moderation. Mediation is another term for indirect effect, and is useful for describing how an effect comes about. One way of describing moderation is to think of the phrase “different slopes for different folks,” a phrase that may be easy to remember if students like classic rock and R&B. The primary method of testing for moderation in multiple regressions is to create a cross product of the two variables that are suspected of interacting and add that cross product sequentially to the regression analysis. A common cause is a variable that affects both a presumed influence and its presumed outcome.
