Contents

250 PART III: Experimental MethodsOVERVIEWKey ConceptIn Chapters 6 and 7 we focused on the basic experimental designs that researchersuse to study the effect of an independent variable. We described how anindependent variable could be implemented with a separate group of participantsin each condition (independent groups designs) or with each participantexperiencing all the conditions (repeated measures designs). We limited ourdiscussion to experiments involving only one independent variable because wewanted you to concentrate on the basics of experimental research. Experimentsinvolving only one independent variable are not, however, the most commontype of experiment in contemporary psychological research. Instead, researchersmost often use complex designs in which two or more independent variablesare studied simultaneously in one experiment.Complex designs can also be called factorial designs because they involvefactorial combination of independent variables. Factorial combination involvespairing each level of one independent variable with each level of a second independentvariable. This makes it possible to determine the effect of each independentvariable alone (main effect) and the effect of the independent variablesin combination (interaction effect).Complex designs may seem a bit complicated at this point, but the conceptswill become clearer as you progress through this chapter. We begin with areview of the characteristics of experimental designs that can be used to investigateindependent variables in a complex design. We then describe the proceduresfor producing, analyzing, and interpreting main effects and interactioneffects. We introduce the analysis plans that are used for complex designs. Weconclude the chapter by giving special attention to the interpretation of interactioneffects in complex designs.DESCRIBING EFFECTS IN A COMPLEX DESIGN• Researchers use complex designs to study the effects of two or moreindependent variables in one experiment.• In complex designs, each independent variable can be studied with anindependent groups design or with a repeated measures design.• The simplest complex design is a 2 2 design—two independent variables,each with two levels.• The number of different conditions in a complex design can be determinedby multiplying the number of levels for each independent variable (e.g.,2 2 4).• More powerful and efficient complex designs can be created by includingmore levels of an independent variable or by including more independentvariables in the design.An experiment with a complex design has, by definition, more than oneindependent variable. Each independent variable in a complex design mustbe implemented using either an independent groups design or a repeatedmeasures design according to the procedures described in Chapters 6 and 7.When a complex design has both an independent groups variable and arepeated measures variable, it is called a mixed design.