In a medical experiment, the experimental group gets the variable being tested--a new drug, for example. However, the control group is given a placebo. It does not contain the variable being tested. By comparing the experimental and control groups, one can gather and analyze data. Eventually, the research team will decide if the treatment was beneficial to the patients based on how well they did compared to the control group which is essentially the non-treated group. In order for research to be valid, an experiment needs to be set up in this way.