Contents

CHAPTER 6: Independent Groups Designs 185OVERVIEWIn Chapter 2 we introduced you to the four goals of research in psychology:description, prediction, explanation, and application. Psychologists use observationalmethods to develop detailed descriptions of behavior, often in naturalsettings. Survey research methods allow psychologists to describe people’sattitudes and opinions. Psychologists are able to make predictions about behaviorand mental processes when they discover measures and observations thatcovary (correlations). Description and prediction are essential to the scientificstudy of behavior, but they are not sufficient for understanding the causes ofbehavior. Psychologists also seek explanation—the “why” of behavior. Weachieve scientific explanation when we identify the causes of a phenomenon.Chapters 6, 7, and 8 focus on the best available research method for identifyingcausal relationships—the experimental method. We will explore how the experimentalmethod is used to test psychological theories as well as to answer questionsof practical importance.As we have emphasized, the best overall approach to research is the multimethodapproach. We can be more confident in our conclusions when we obtaincomparable answers to a research question after using different methods. Ourconclusions are then said to have convergent validity. Each method has differentshortcomings, but the methods have complementary strengths that overcomethese shortcomings. The special strength of the experimental method is thatit is especially effective for establishing cause-and-effect relationships. In thischapter we discuss the reasons researchers conduct experiments and we examinethe underlying logic of experimental research. Our focus is on a commonlyused experimental design—the random groups design. We describe the proceduresfor forming random groups and the threats to interpretation that applyspecifically to the random groups design. Then we describe the proceduresresearchers use to analyze and interpret the results they obtain in experiments,and also explore how researchers establish the external validity of experimentalfindings. We conclude the chapter with consideration of two additional designsinvolving independent groups: the matched groups design and the naturalgroups design.WHY PSYCHOLOGISTS CONDUCT EXPERIMENTS• Researchers conduct experiments to test hypotheses about the causes ofbehavior.• Experiments allow researchers to decide whether a treatment or programeffectively changes behavior.One of the primary reasons that psychologists conduct experiments is tomake empirical tests of hypotheses they derive from psychological theories.For example, Pennebaker (1989) developed a theory that keeping in thoughtsand feelings about painful experiences might take a physical toll. Accordingto this “inhibition theory,” it’s physically stressful to keep these experiencesto oneself.