The experimental group of study participants, also called the treatment group, is comprised of those individuals who will receive intervention and whose outcomes will be observed, measured and compared to the control group. Unlike the experimental group, the control group will receive no intervention. Both the experimental and the control group should be of similar composition and subject to the same external factors to enable the study to most realistically reflect the differences between intervention and nonintervention.
In some studies, the control group may believe they are receiving an intervention, or treatment, but they are instead receiving a placebo. An example would be a study in which both the experimental and the control group receive a pill believed to be a new type of drug, but the control group is instead given a sugar pill, or placebo, that appears to be identical to the actual medication being studied. The use of a placebo helps to minimize the possibility of subject bias, in which members of either the experimental or the control group are led to believe in or anticipate a particular or preferred form of outcome.
Experimenter bias can also play a role in affecting the outcomes of the experimental and control groups. The experimenter may be hoping for a particular result to be obtained. This bias can possibly result in the experimenter giving subtle hints to the participants or allowing members of the preferred-outcome group some extra time to answer questions. Experimenter bias and subject bias can be minimized in a double-blind study in which both the experimenters and the entirety of the participants remain unaware of who is receiving the intervention and who is getting the placebo.