A method used to prevent bias in treatment studies. In a single-blind study, the patient is not told whether he/she is taking the standard treatment or the new treatment being tested. Only the doctor knows.

A research study in which the participants (or subjects) do not know which group or arm of the study they are in. For example, in a single blind study comparing an experimental intervention to the current standard intervention, the subjects do not know whether they are receiving the experimental intervention or the standard intervention. A study is "blinded" so that the subjects' expectations about the intervention will be less likely to affect the outcome, and the results will be more likely to be unbiased. A blinded study may also be called a "masked study." See also DOUBLE-BLIND STUDY.

One in which subjects do not know whether they are receiving the active drug or a placebo.