Supplementing Interaction: Simple Effects (2 of 3)

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A significant main effect of condition indicates that, on average, the treatment condition leads to better performance than does the control condition. Since the effect of condition is different for the two treatments, a significant main effect of condition does not necessarily imply an effect of condition for both tasks. It might be that, in the population, there is an effect of condition for Task 1 but not for Task 2. If the researcher wished to know whether there were an effect of condition for both tasks, he or she could not rely on the test of the main effect. Instead, the researcher would test the significance of the effect of condition separately for the two tasks. The effect of a variable at a specific level of another variable is called a "simple effect" of the variable. In this example there are two simple effects of condition: the effect of condition for Task 1 and the effect of condition for Task 2. The presence of interaction means that the main effect is not representative of the simple effects. If you wish to know whether a variable has an effect at each level of a second variable, you should test the simple effects.