In a scientific experiment, the control is separated from the rest of the experiment so that the independent variable that is being tested is unable to influence the results. This segment is often known as the "control group."
An example of a scientific experiment with a control is a trial of a new medication. The patients in the drug trial are given dosages of what they are told is a medication and are asked to report how well the medication treats their condition or whether they experience any side effects. However, some proportion of the patients are in the control group; these patients are given a placebo instead of the medication being tested. A comparison of the data between the control group and the patients who received the drug reveals the actual effects of the drug.