Simple Effects, Simple Contrasts, and Main Effect Contrasts

Simple Effects Following a significant interaction, follow-up tests are usually needed to explore the exact nature of the interaction. Simple effects (sometimes called simple main effects) are differences among particular cell means within the design. More precisely, a simple effect is the effect of one independent variable within one level of a second independent variable. In our hypothetical example of the eyewitness study, one simple effect might involve the effect of violence only for individuals within the no training condition. Similar to the contrasts following a significant one-way ANOVA, the simple effect test uses the error term and df from the whole design. Remember that when the error term (MSs/A) is derived using all of the cases in the study, it will be generally be smaller and will result in greater statistical power. Thus, testing simple effects is preferable to using t-tests.