interaction effect

Definition:
Interaction effect: An effect that occurs in randomized factorial design when the influence of one factor depends on the level or category of a second factor (e.g., a treatment affects males but not females). The key word here is depends.

Notes

You may be familiar with the concept of interaction already, possibly in the form of warnings about drug interactions. This occurs if two drugs—each separately having a small effect—when combined have an amplifying effect. For example, some medications may make you slightly sleepy; so might a glass of wine. The combination of medication and wine, however, may make you very sleepy (more than the slight effects merely added together)—an interaction effect. (Suter 2012, pg. 14)

In a sense, an interaction occurs when the total effect is more than the sum of its parts. Interaction can also work in the opposite way—each of two drugs may have a small effect, but the two drugs taken together may have no overall effect. Here the total is less than the sum of its parts. In this sense, it is more accurate to say that an interaction occurs when the total is different from the sum of its parts.

Interaction effects occur when the influence of one factor depends on the category of the other factor ( e.g., the treatment affects females more than males ). (pg. 16)