Main Effects and Interaction Effects of Factorial Design
MAIN EFFECTS AND INTERACTION EFFECTS OF FACTORIAL DESIGN
Two kinds of information can be gleaned from a factorial design. The first piece of information gained from a factorial design is whether there are any main effects. A main effect is an effect of a single independent variable. In our design with two independent variables, two main effects are possible: an effect of word type and an effect of rehearsal type. In other words, there can be as many main effects as there are independent variables. The second piece of information is whether or not there is an interaction effect. As the name implies, this is information regarding how the variables or factors interact. Specifically, an interaction effect indicates the effect of each independent variable across the levels of the other independent variable. When there is an interaction between two independent variables, the effect of one independent variable depends on the level of the other independent variable.
Typically, researchers begin by assessing whether or not there is an interaction effect because having an interaction effect indicates that the effect of one independent variable depends on the level of the other independent variable. However, when first beginning to interpret two-way designs, students usually find it easier to begin with the main effects and then move on to the interaction effect. What we need to keep in mind is that if we later find an interaction effect, any main effects will have to be qualified. Remember, because we have two independent variables, there is the possibility for two main effects—one for word type and one for rehearsal type. The main effect of each independent variable tells us about the relationship between that single independent variable and the dependent variable. At face value, the main effects tell us that, overall, subjects recall more words when they are concrete and when imagery rehearsal is used. However, we now need to assess whether there is an interaction
Two kinds of information can be gleaned from a factorial design. The first piece of information gained from a factorial design is whether there are any main effects. A main effect is an effect of a single independent variable. In our design with two independent variables, two main effects are possible: an effect of word type and an effect of rehearsal type. In other words, there can be as many main effects as there are independent variables. The second piece of information is whether or not there is an interaction effect. As the name implies, this is information regarding how the variables or factors interact. Specifically, an interaction effect indicates the effect of each independent variable across the levels of the other independent variable. When there is an interaction between two independent variables, the effect of one independent variable depends on the level of the other independent variable.
Typically, researchers begin by assessing whether or not there is an interaction effect because having an interaction effect indicates that the effect of one independent variable depends on the level of the other independent variable. However, when first beginning to interpret two-way designs, students usually find it easier to begin with the main effects and then move on to the interaction effect. What we need to keep in mind is that if we later find an interaction effect, any main effects will have to be qualified. Remember, because we have two independent variables, there is the possibility for two main effects—one for word type and one for rehearsal type. The main effect of each independent variable tells us about the relationship between that single independent variable and the dependent variable. At face value, the main effects tell us that, overall, subjects recall more words when they are concrete and when imagery rehearsal is used. However, we now need to assess whether there is an interaction