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192 PART III: Experimental Methodsthe conditions. The random groups design used by Dittmar et al. (2006) may bedescribed as follows:Stage 1 Stage 2 Stage 3R 1 X 1 O 1R 2 X 2 O 1R 3 X 3 O 1where R 1 , R 2 , and R 3 refer to the random assignment of subjects to the three independentconditions of the experiment; X 1 is one level of an independent variable(e.g., Barbie), X 2 is a second level of the independent variable (e.g., Emme), and X 3is a third level of the independent variable (e.g., neutral images). An observationof behavior (O 1 ) in each group is then made.In the Dittmar et al. (2006) study of girls’ body image, if participantsviewing the Barbie images were shown to be more overweight or to ownmore Barbie dolls than participants viewing the Emme or neutral images,a plausible alternative explanation for the findings exists. It’s possible thatbeing overweight or having more Barbie dolls, not the version of the images,could explain why participants in the Barbie condition experienced greaterbody dissatisfaction. (In the language of the researcher, a confounding wouldbe present.) Similarly, individual differences in the girls’ body dissatisfactionbefore the experiment was conducted could be a reasonable alternativeexplanation for the study’s findings. When random assignment is used tobalance these individual differences across the groups, however, we can logicallyrule out the alternative explanation that any differences we obtain betweenthe groups on the dependent variable are due to characteristics of theparticipants.When we balance a factor such as body weight, we make the three groupsequivalent in terms of their average body weight. Note that this differs fromholding body weight constant, which would require that all of the girls in thestudy have the same body weight. Similarly, balancing the number of Barbiedolls owned by girls in the three groups would mean that the average numberof dolls owned in the three groups is the same, not that the number of dollsowned by each girl is held constant at some number. The beauty of random assignmentis that all individual differences are balanced, not just the ones we’vementioned. Therefore, we can rule out alternative explanations due to any individualdifferences among participants.In summary, Dittmar and her colleagues concluded that exposure to thinbody images, such as Barbie, causes young girls to be dissatisfied with their ownbodies. They were able to make this conclusion because they• manipulated an independent variable that varied the images girls viewed,• eliminated other plausible explanations through holding relevantconditions constant, and• balanced individual differences among the groups through randomassignment to conditions.Box 6.1 summarizes how Dittmar and her colleagues applied the experimentalmethod, specifically, the random groups design, to their study of young girls’body image.