Correlational studies are not uncommon in psychological research. Often, however, a researcher wants even more specific information about the relationships among variables—in particular, about whether one variable causes a change in another variable. In such a situation, experimental research is warranted. This drawback of the correlational approach—its inability to establish causal relationships—is worth considering for a moment. In the hypothetical study described above, the researcher may find that viewing considerable television violence predicts high levels of aggressive behavior, yet she cannot conclude that these viewing habits cause the aggressiveness. After all, it is entirely possible that aggressiveness, caused by some unknown factor, prompts a preference for violent television. That is, the causal direction is unknown; viewing television violence may cause aggressiveness, but the inverse (that aggressiveness causes the watching of violent television programs) is also feasible.
As this is a crucial point, one final illustration is warranted. What if, at a certain Rocky Mountain university, a correlational study has established that high levels of snowfall predict low examination scores? One should not conclude that something about the chemical composition of snow impairs the learning process. The correlation may be real and highly predictive, but the causal culprit may be some other factor. Perhaps, as snowfall increases, so does the incidence of illness, and it is this variable that is causally related to exam scores. Maybe, as snowfall increases, the likelihood of students using their study time for skiing also increases.
Experimentation is a powerful research method because it alone can reveal cause-effect relationships. In an experiment, the researcher does not merely measure the naturally occurring relationships between variables for the purpose of predicting one from the other; rather, he or she systematically manipulates the values of one variable and measures the effect, if any, that is produced in a second variable. The variable that is manipulated is known as the independent variable; the other variable, the behavior in question, is called the dependent variable (any change in it depends upon the manipulation of the independent variable). Experimental research is characterized by a desire for control on the part of the researcher. Control of the independent variable and control over extraneous variables are both wanted. That is, there is a desire to eliminate or hold constant the factors (control variables) other than the independent variable that might influence the dependent variable. If adequate control is achieved, the researcher may be confident that it was, in fact, the manipulation of the independent variable that produced the change in the dependent variable.
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