In a simple blind study, the project leader knows which drug I am taking. There is a good chance that the effect of the drug on me is triggered by the placebo effect. But why is my placebo effect alleviated if the project leader does not know what I am taking either?
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If the researcher knows whether you got the real stuff or the placebo, they can (either consciously or subconsciously) convey that information to the subject….thus removing the “blind”.
There are a number of non-verbal ways people communicate information: body language, posture, voice intonation, editorializing or word choice, etc.
That’s why double blind is considered the gold standard. The researcher can’t inadvertently tip off the subject, because they don’t know.
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