Which procedure ensures that neither the participants nor the researchers know who has received the treatment or placebo?

The double-blind procedure is designed to eliminate bias in clinical trials and research studies by ensuring that neither the participants nor the researchers are aware of who is receiving the treatment and who is receiving a placebo. This approach is crucial in objective assessment of the intervention's efficacy, as it prevents the expectations or beliefs of either group from influencing the outcomes. When both groups are blind to the treatment conditions, the results are more likely to reflect the true effects of the treatment itself, rather than being skewed by psychological factors such as the placebo effect or experimenter bias.

In contrast, single-blind procedures only keep the participants unaware of their treatment assignment while the researchers know which participants are receiving the treatment or placebo. Triple-blind procedures extend this approach by also keeping the individuals analyzing the data unaware, but it is the double-blind design that specifically addresses the question of ensuring that both participants and researchers are blind to treatment assignment. Open-label trials, where all parties are aware of the treatment assignments, lack the key element of blinding, making them less effective in reducing bias.