A research design where neither participants nor researchers know who is receiving the experimental treatment versus the control condition. This method prevents both participant and experimenter expectations from influencing the results.
The term 'double-blind' emerged in medical research in the 1940s, combining 'double' (meaning both parties) with 'blind' (meaning unaware). It evolved from earlier 'single-blind' studies where only participants were kept unaware, as researchers recognized that investigator bias was equally problematic.
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