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In this case, it's better if researchers do not know which treatment the

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participants received. This is called a double blind experiment, because neither

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the participants nor the researchers studying them know which treatment they

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received. Single blind is when only the participants are unaware of their

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treatment condition. And the reason this experiment is double blind is because

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if the researcher knows which treatment each person received, that may impact

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their judgements on how well the active pill works versus the Placebo. It may

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subconsciously cause the researchers to think that the people who took the

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active pill are sleeping better, and this might consciously or unconsciously

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alter their measurements.