What type of study is conducted when patients are assigned to a treatment group and a placebo group to assess the efficacy of an intervention?

• A. Case-control
• B. Cohort
• C. Controlled trial
• D. Crossover

Answer: (C)

The study being described is a controlled trial, which is designed specifically to evaluate the efficacy of an intervention by comparing outcomes between a group receiving the treatment and a group receiving a placebo. This approach allows for a direct assessment of the intervention's effect while controlling for potential confounding variables. The random assignment to either the treatment or placebo group helps to ensure that any differences observed in outcomes can be attributed to the intervention itself rather than other factors. This design is fundamental in clinical research, as it provides robust evidence regarding the safety and effectiveness of new therapies.

In contrast, other study types like case-control studies focus on comparing subjects with a particular outcome to those without it, retrospective in nature and not suitable for assessing new interventions. Cohort studies evaluate outcomes in groups of subjects over time but do not involve randomization to treatment and placebo. Crossover trials involve participants receiving both the treatment and placebo in different phases of the study, but this is not the same as assigning two distinct groups at the start. This distinct design characteristic of a controlled trial allows researchers to draw clear conclusions about the intervention's effect.