main effect is the effect of one of just one of the independent variables on the dependent variable. There will always be the same number of main effects as independent variables. An interaction effect occurs if there is an interaction between the independent variables that affect the dependent variable.For example, if a researcher is studying how gender (female vs. ... Diet B) influence weight loss, an interaction effect would occur if women using Diet A lost more weight than men using Diet A. Interaction effects contrast with gender and may obscure the female vs diet b. See also higher order interaction.