a study in which the investigator, and not the patient, knows which study patients are taking placebo and which are taking the drug being tested.

Researcher knows placebo condition, but participant does not.

A double-blind study, in which neither subject nor evaluator knows what treatment or regime has been administered, reduces the risk of bias (psychological or otherwise) being introduced by either the investigator or the subjects of the study. Single-blind occurs when one of these two is aware of the threatment or regime administered. See also Blind study, Blinded evaluation, Double-blind.