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Two advantages of the experimental research design are (1) the assurance that the independent variable (also
known as the experimental manipulation) occurs prior to the measured dependent variable, and (2) the creation of
initial equivalence between the conditions of the experiment (in this case by using random assignment to
conditions).
Experimental designs have two very nice features. For one, they guarantee that the independent variable occurs prior
to the measurement of the dependent variable. This eliminates the possibility of reverse causation. Second, the
influence of common-causal variables is controlled, and thus eliminated, by creating initial equivalence among the
participants in each of the experimental conditions before the manipulation occurs.
The most common method of creating equivalence among the experimental conditions is
through random assignment to conditions, a procedure in which the condition that each participant is assigned to is
determined through a random process, such as drawing numbers out of an envelope or using a random number
table. Anderson and Dill first randomly assigned about 100 participants to each of their two groups (Group A and
Group B). Because they used random assignment to conditions, they could be confident that, before the experimental
manipulation occurred, the students in Group A were, on average, equivalent to the students in Group B on every
possible variable, including variables that are likely to be related to aggression, such as parental discipline style, peer
relationships, hormone levels, diet—and in fact everything else.
Then, after they had created initial equivalence, Anderson and Dill created the experimental manipulation—they had
the participants in Group A play the violent game and the participants in Group B play the nonviolent game. Then
they compared the dependent variable (the white noise blasts) between the two groups, finding that the students who