In the design of experiments, a main effect is the effect of an independent variable on a dependent variable averaging across the levels of any other independent variables. The term is frequently used in the context of factorial designs and regression models to distinguish main effects from interaction effects.

For example, in factorial designs, the main effect is what the independent variables elicit when averaged out over each other.

References

• McBurney, D.M., White, T.L. (2004). Research Methods. CA: Wadsworth Learning.
• Mook, Douglas G. (2001). Psychological Research: The Ideas Behind the Methods. NY: W. W. Norton & Company.