THE CLASSICAL EXPERIMENT
    Independent and Dependent Variables
    Pretesting and Posttesting
    Experimental and Control Groups
    The Double-Blind Experiment

    Experiments are designed to determine the presence or absence of causal relationships: whether a particular independent variable causes (influences, impacts, determines) a particular dependent variable.  Does viewing a film on "Women in Science" reduce prejudice against women?  In this case, prejudice is the dependent variable.  Viewing the film (or not) is the independent variable, called the stimulus in experimental designs.

    To determine whether showing the film reduces prejudice, we must measure levels of prejudice before and after: known as pretesting and posttesting.

    To be sure that it was the film that caused any reductions in prejudice, we divide the subjects into two groups: (1) an experimental group to whom we administer the stimulus (show the film) and a control group who do not receive the stimulus.  If both groups have a reduction of prejudice, it would not appear that the film caused the difference, but if only the experimental group has reduced prejudice, that supports the possibility that the film makes a difference.

    If subjects know they are receiving some treatment, they commonly change based on that expectation (e.g., medical patients improve if they think they are receiving a powerful new drug).  This is known as the placebo effect, and subjects are never told whether they are in the experimental or control group.  Whenever the dependent variable is measured subjectively (e.g., doctors deciding whether patients have improved), there is a risk that their judgments will be affected by expectations about the effect of the stimulus.  In a double-blind experiment, neither the subjects nor the experimenters making judgment measurements know whether subjects are in the experimental or control group.