ABSTRACT
This chapter first describes variations on the basic experimental design with a single independent variable. Variations include the pretest-posttest control group design, the posttest-only control group design, and the Solomon four-group design. It then moves to discussion of factorial designs that combine more than one independent variable with multiple experimental treatments. Factorial designs allow for testing interactions between two or more independent variables, evidenced if the effect of one independent variable on the dependent variable is altered or moderated by variations in the level(s) of other independent variable(s). Different forms of interaction effects are described and interpreted. The chapter then discusses the use and implementation of blocked designs, where experimentally manipulated independent variable(s) are combined with nonexperimental factor(s) (usually individual difference variables, such as sex or political affiliation). The chapter concludes with discussion of how to design and conduct experiments with repeated measures, where participants are exposed to more than one treatment condition of the independent variable.
Chapter 2 introduced the concepts of internal and external validity (two important forms of validity to be considered in experiments) and provided some information on the basic structure of an experiment. Here, we expand on this basic structure to consider variations in the ways in which experiments can be designed, set up, and executed.
