In a double-blind design, which statement is true?

• A. Both participants and researchers know treatment.
• B. Only participants know the assignment.
• C. Neither participants nor researchers know the treatment assignment.
• D. The assignment is revealed after the study ends.

Answer: (C)

Double-blind design aims to minimize bias by keeping the treatment allocation hidden from both the participants and the researchers who interact with them or assess outcomes. Because neither party knows who is getting the active treatment versus a control, expectations and beliefs can’t unconsciously influence how participants report symptoms or how outcomes are measured. That’s why the statement that neither participants nor researchers know the assignment is the true one.

The other ideas don’t capture the same protective mechanism: if both groups knew the treatment, bias could creep in from expectations; if only participants knew, researchers could still introduce bias in data collection or interpretation; and while some trials allow unblinding after data collection for safety or analysis, the defining feature of a double-blind study is concealment during the study, not the timing of any later unblinding.