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A double-blind research study is one in which neither the subject nor the investigator knows what treatment (if any) the subject is receiving. Only after all the data are collected is the investigator made aware of which subjects received which treatments.

The purpose of keeping the subject unaware of the treatment is to minimize the psychological effects of drug treatment. For example, a subject who knows she is being given an experimental drug may expect beneficial results and this may in fact increase the efficacy of the drug. (See also: placebo effect.)

The purpose of keeping the investigator unaware of the treatment is to minimize any inadvertent bias by the investigator. For example, if the investigator believes that a new drug is going to be highly effective in improving memory, he may inadvertently tend to report data that support his belief. By keeping the investigator unaware of treatment until the end of the study, this source of potential bias is eliminated.

Well-designed research studies, particularly clinical trials of new drugs, usually employ randomization, placebo controls and double-blind techniques.

Further Reading: "Putting Ginkgo to the Test"