The experimenter might want to control or hold constant other variables. Perhaps she suspects that age, social class, ethnicity, and gender could also influence the children's aggressiveness. She might want to make sure that these subject variables are eliminated by either choosing subjects who are alike in these ways or by making sure that the groups are balanced for these factors (for example, equal numbers ofboys and girls in each group). There are numerous other extraneous variables that might concern the researcher, including the time of day when the children participate, the length of time between television viewing and the assessment of aggressiveness, the children's diets, the children's family structures (single versus dual parent, siblings versus only child), and the disciplinary styles used in the homes. Resource limitations prevent every extraneous variable from being controlled, yet the more control, the more confident the experimenter can be of the cause-effect relationship between the independent and dependent variables.
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