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Masking, commonly referred to as "blinding," is a method of concealing knowledge of treatment assignment as a means of reducing bias in reporting or measuring outcome on the part of patients, those collecting the data, or those analyzing study results. It has been demonstrated that when patients or investigators know the treatment assignment (or even just when they think they know it), they may introduce their own prejudices about the effect of treatment into the assessment of outcome. If masking is successful, patients and/or investigators will not know what treatment was assigned. In a "single masked" study, usually the patient is masked with regard to treatment; in a "double masked" study, the patient and the investigator are masked; "triple masking" refers to the study in which the data monitors are also unaware of treatment assignment (more controversial).