Treatments, Controls, and Placebos
Treatment and Control Groups
In some experiments, the independent variable is whether or not subjects receive some treatment. The treatment could be a medical treatment, psychotherapy, or almost anything else. For example, it could be whether or not college students take a weekly quiz. In these experiments, the group that gets the treatment is referred to as the treatment group or the experimental group. The group that does not get the treatment is referred to as the control group.
What is the purpose of the control group? Remember that claims about treatment effectiveness are claims about relationships, so they require a comparison of one variable across levels of the other. What are the levels of the independent variable? Receiving the treatment is one … and not receiving the treatment is the other. The dependent variable must be compared across those two levels. Another way to look at this is that you always have to worry about whether subjects who do not receive the treatment might behave exactly like subjects who did. The only way you can know for sure it to include a control group.
For example, imagine that a college professor gives a group of students a weekly quiz, and he finds that on average their final course grade is a B+. Is this evidence that taking the quizzes had a positive effect on students’ performance in the course? Of course not! You have to know the average grade of a group of students who did not take a weekly quiz. If it is lower then a B+, then you have evidence that the treatment worked. If it is also a B+, then the treatment did not work! (If it is higher than a B+, then you actually have evidence that the weekly quiz hurts students’ performance.)
Does There Have to Be a Control Group?
Sometimes students get confused and think that every experiment must include a control group. It is true that every experiment must include a comparison across levels of the independent variable, but it is not true that one of the conditions must be a control group. Consider, for example, an experiment on the effects of noise level on concentration. The experimenter might test people under a quiet condition and under a noisy condition, but she does not necessarily have to test them under a condition with absolutely no noise at all. (It is not even clear that this would be possible.) Similarly, a college professor might compare the grades of students who have taken difficult weekly quizzes and students who have taken easy weekly quizzes, but he does not necessarily have to compare these students to another group that takes no quizzes at all. In both cases, the control condition might be interesting, but it is not necessary to draw conclusions about a causal relationship between the variables.
Placebos and Placebo-Control Conditions
A placebo is a fake treatment that appears to be a real treatment from the subject’s perspective. For example, in studies of drug effectiveness, some subjects might be given a placebo that is nothing more than a sugar pill. In studies of psychotherapy effectiveness, some subjects might come in and talk to a therapist for an hour even though the therapist does not use the technique of interest.
In many experiments, subjects in the control condition are given a placebo. This is sometimes called a placebo control condition to distinguish it clearly from a no-treatment control condition (i.e., the kind of control condition discussed above).
The reason for doing this is as follows. Subjects in a treatment condition are often expecting an effect of the treatment, whereas subjects in a no-treatment control condition are not. That is, subjects’ expectations are a potential confounding variable, so if there is a difference between treatment and no-treatment control conditions, it could be caused by differences in subjects’ expectations rather than by the treatment itself. This is called a placebo effect. But note that if subjects in the control group receive a placebo, then they have the same expectations as subjects in the treatment group. Subjects expectations have been controlled. So now any difference between the two groups must be due to the treatment itself and not to subjects’ expectations.
Many experiments are designed with both a no-treatment control group and a placebo control group. Comparing these two groups allows the researchers to see whether there is a placebo effect. Comparing the placebo control group to the treatment group allows them to see whether the treatment had an effect over and above that of the placebo.