A randomized controlled trial is a special method of doing a scientific experiment which can reduce certain sources of bias. It is often used in the context of testing whether drugs are effective against a set of symptoms. When doing the test, the participants are randomly put into different groups. Each group is treated differently, and at the end the results are compared. A common case is that one group will receive a placebo drug, while the other will receive the real one. The trial is called blinded, if the participants, or those giving the drugs do not know what group a patient is in.