APA Dictionary of Psychology

in an analysis of variance, a test used to determine the effect of one independent variable on the dependent variable at a single level of a second independent variable; that is, the test is used to examine the effects of one of the independent variables with the other independent variable held constant. For example, consider a researcher who finds a statistically significant interaction between two independent variables, A and B, that each have two levels, A₁ and A₂ and B₁ and B₂. In this case, a test of simple effects involves two statistical tests: one of the difference between A₁ and A₂ at B₁ and a second of the difference between A₁ and A₂ at B₂. The comparisons can also be undertaken by examining the difference between B₁ and B₂ at A₁ and the difference between B₁ and B₂ at A₂.