Returning to the relationship between television viewing habits and aggressive behavior in children, suppose that correlational evidence indicates that high levels of the former variable predict high levels of the latter. Now the researcher wants to test the hypothesis that there is a cause-effect relationship between the two variables. She decides to manipulate exposure to television violence (the independent variable) to see what effect might be produced in the aggressiveness of her subjects (the dependent variable). She might choose two levels of the independent variable and have twenty children watch fifteen minutes of a violent detective show, while another twenty children are subjected to thirty minutes of the same show.
Ifan objective rating ofplayground aggressiveness later reveals more hostility in the thirty-minute group than in the fifteen-minute group, she still cannot be confident that higher levels of television violence cause higher levels ofaggressive behavior. More information is needed, especially with regard to issues of control. To begin with, how does the researcher know that it is the violent content of the program that is promoting aggressiveness? Per haps it is the case that the more time they spend watching television, regardless of subject matter, the more aggressive children become.
This study needs a control group: a group of subjects identical to the experimental subjects with the exception that they do not experience the independent variable. In fact, two control groups might be employed, one that watches fifteen minutes and another that watches thirty minutes of nonviolent programming. The control groups serve as a basis against which the behavior of the experimental groups can be compared. If it is found that the two control groups aggress to the same extent, and to a lesser extent than the experimental groups, the researcher can be more confident that violent programming promotes relatively higher levels of aggressiveness.
The experimenter also needs to be sure that the children in the thirty-minute experimental group were not naturally more aggressive to begin with. One need not be too concerned with this possibility if one randomly assigns subjects to the experimental and control groups. There are certainly individual differences among subjects in factors, such as personality and intelligence, but with random assignment (a technique for creating groups of subjects across which individual differences will be evenly dispersed) one can be reasonably sure that those individual differences are evenly dispersed among the experimental and control groups.
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