ABSTRACT

Good experimental design is characterized by careful attention to the control of real extraneous variables and the investigation of important topics. Even the most exquisitely crafted experiment cannot save a stupid idea, and a brilliant idea is wasted in an experiment with a serious design problem. Sound judgment combined with knowledge can improve an experimental design so that potential extraneous variables are manageable, thus allowing the experimenter to control only those variables that can be reasonably assumed to influence the dependent variable. This chapter is primarily concerned with controlling extraneous variables that occur when experimenters manipulate the independent variable. A related problem deals with ensuring that the various treatment groups have the same participant characteristics. Holding conditions constant is essential to good experimental design. Researchers will also frequently add treatment groups to an experimental design to ensure that the results are not caused by an extraneous variable.