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CHAPTER 6: Independent Groups Designs 191experiment and (2) body dissatisfaction came after viewing the images (timeorderrelationship). The third requirement for causal inference, elimination ofalternative explanations, was accomplished in this experiment through holdingconditions constant and balancing.Holding Conditions Constant In Dittmar et al.’s experiment, several factors thatcould have affected the girls’ attitudes toward their body were kept the sameacross the three conditions. All of the girls heard the same story about shoppingand attending a birthday party, and they looked at their picture books forthe same amount of time. They all received the same instructions throughoutthe experiment and received the exact same questionnaire at the conclusion.Researchers use holding conditions constant to make sure that the independentvariable is the only factor that differs systematically across the groups.If the three groups had differed on a factor other than the picture books, thenthe results of the experiment would have been uninterpretable. Suppose theparticipants in the Barbie condition had heard a different story, for example,a story about Barbie being thin and popular. We wouldn’t know whether theobserved difference in the girls’ body dissatisfaction was due to viewing theimages of Barbie or to the different story. When the independent variable ofinterest and a different, potential independent variable are allowed to covary,a confounding is present. When there are no confoundings, an experiment hasinternal validity.Holding conditions constant is a control technique that researchers use toavoid confoundings. By holding constant the story the girls heard in the threeconditions, Dittmar and her colleagues avoided confoundings by this factor.In general, a factor that is held constant cannot possibly covary with the manipulatedindependent variable. More importantly, a factor that is held constantdoes not change, so it cannot possibly covary with the dependent variableeither. Thus, researchers can rule out factors that are held constant as potentialcauses for the observed results.It is important to recognize, however, that we choose to control only thosefactors we think might influence the behavior we are studying—what weconsider plausible alternative causes. For instance, Dittmar et al. held constantthe story the girls heard in each condition. It is unlikely, however, that theycontrolled factors such as the room temperature to be constant across the conditionsbecause room temperature probably would not likely affect body image(at least when varying only a few degrees). Nevertheless, we should constantlyremain alert to the possibility that there may be confounding factors in ourexperiments whose influence we had not anticipated or considered.Balancing Clearly, one key to the logic of the experimental method is formingcomparable (similar) groups at the start of the experiment. The participants ineach group should be comparable in terms of various characteristics such astheir personality, intelligence, and so forth (also known as individual differences).The control technique of balancing is required because these factors often cannotbe held constant. The goal of random assignment is to establish equivalentgroups of participants by balancing, or averaging, individual differences across