Which study design involves alternating between active treatment and placebo?

• A. Placebo
• B. Phase III Testing
• C. Crossover Study
• D. NDA

Answer: (C)

The idea being tested is a crossover study design. In this approach, each participant experiences both conditions—active treatment and placebo—across separate time periods, with a washout interval in between to prevent carryover effects. Because the same person acts as their own control, variability from individual differences is reduced, making it easier to detect a true treatment effect and often allowing the study to enroll fewer participants.

This alternating structure is what distinguishes crossover studies from other designs. A placebo by itself is just a control condition, not a study design. Phase III testing refers to a late-stage trial phase that typically uses parallel-group designs to compare treatments, and an NDA is a regulatory filing, not a design. Keep in mind that crossover designs work best when the condition is stable and the treatment effect is reversible; long-lasting effects or progressive conditions can complicate or invalidate the crossover approach.