In a trial, a group of people who are suffering from a disease are given a particular treatment. To determine the effectiveness of the treatment, a controlled trial is performed. Two groups are studied: onc group (the study group) is given the treatment and the other (the control group) is not. The controls may be given a placebo - something which seems to be identical to the treatment but which has no effect. If there is an equal possibility that patients may be selected for the study group or for the controls, the trial is said to be randomized. A randomized controlled trial makes bias - error in a study which influences the results - less likely. An additional way of removing bias is blinding: patients do not know if they are receiving the treatment or the placebo. If, in addition, the researchers do not know who is receiving the treatment, the trial is a double blind trial. Randomized controlled trials are used to test treatments or preventive measures.

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