Control
A controlled study uses a control group. This is a group of subjects who are as similar as possible to the experimental group -- i.e., the study subjects who are given the treatment or subjected to the conditions being studied -- but who are not subjected to the experiment. They are often given a placebo. This corrects for the placebo effect and allows researchers to tell whether the actual treatment is having an effect. It also increases the likelihood that any changes in the experimental group are caused by the treatment and not by external factors.