Which statement defines a randomized controlled trial?

• A. A cross-sectional survey
• B. A descriptive study of a single patient
• C. An observational study without randomization
• D. An experiment with random assignment to control and experimental groups

Answer: (D)

Randomized controlled trials are experiments where participants are assigned to groups by randomization, with one group receiving the intervention and the other serving as a control. This random assignment helps balance both known and unknown factors between groups, reducing bias and allowing a fair comparison of outcomes. The control group provides a baseline to determine what would happen without the intervention, while randomization strengthens the ability to infer causality—that any observed effect is due to the intervention rather than other influences. The other designs described lack this combination: a cross-sectional survey measures data at one moment without any intervention; a descriptive study of a single patient (case description) has no comparison group; and an observational study without randomization observes outcomes without assigning an intervention. Hence, the scenario that features random assignment to control and experimental groups best defines a randomized controlled trial.