A clinical trial is used to evaluate the effectiveness and safety of medications or medical devices. Effectiveness of a drug or device within a clinical trial is measured by monitoring the effects on a large groups of people, known as a pool or segment. In clinical trials volunteers are often used, but in some cases, trial members may be paid for their participation. Participants, known as subjects, are often divided in to two or more groups, including a control group (known as a placebo) that does not receive the experimental treatment. Effects are measured between groups, including the placebo, to determine effects, if any.