A double blind study is one in which neither the patient nor the physician knows whether the patient is receiving the treatment of interest or the control treatment.
For example, studies of treatments that consist essentially of taking pills are very easy to do double blind - the patient takes one of two pills of identical size, shape, and color, and neither the patient nor the physician needs to know which is which.
A double blind study is the most rigorous clinical research design because, in addition to the randomization of subjects which reduces the risk of bias, it standardizes the placebo effect which is a further challenge to the validity of a study.
The placebo effect could be thought of in this way:
1. Patients who believe they are receiving a new experimental treatment tend to be more optimistic about the outcome. This means that, when asked, they tend to minimize health problems and give more weight to positive effects. They also tend to take better care of themselves and comply better with the conditions of the experiment. There is also substantial evidence that, independent of all this, patients who have positive beliefs about their treatment do better than patients who do not. In many situations, the placebo effect is at least as strong as any objective effects of the treatment!
2. Doctors who believe that a patient is receiving a new experimental treatment tend to be more optimistic about that patient's chances, evaluate their state of health more favorably, and communicate positive expectations to the patients, who in turn try to get better so as to prove their doctor right!
Adapted from SUNY Downside Medical Center/Medical Research Library of