IT IS A commonly accepted notion among some clinicians and most clinical investigators that the doctors' belief system, ie, his conviction as to the efficacy of his treatment and the treatability of his patient, can influence the outcome of treatment.1-4 Despite some notes of reservation5 the double-blind design has been considered an adequate means of controlling the effect of the doctors' belief system on outcome.6-8 While it may be true that the double-blind keeps the doctor in the dark, it by no means guarantees a state of clinical neutrality and intellectual stasis. The double-blind design cannot stop the doctor from attempting to guess which active agent his patient is receiving, or if his patient is receiving an active agent at all. The doctors' guess may thus introduce a systematic bias which can nullify the intent of the double-blind design.
Engelhardt DM, Margolis RA, Rudorfer L, Paley HM. Physician Bias and the Double-Blind. Arch Gen Psychiatry. 1969;20(3):315-320. doi:10.1001/archpsyc.1969.01740150059009