Which design evaluates two or more treatments simultaneously by varying combinations of treatments?

• A. Parallel Group Design
• B. Crossover Design
• C. Factorial Designs
• D. Multicenter Trials

Answer: (C)

Factorial designs evaluate two or more treatments at once by varying combinations of treatments across groups. Each treatment factor has levels (for example, present vs absent or low vs high dose), and the study includes all possible combinations of those levels. This structure lets you estimate the main effect of each treatment and also whether the treatments interact with each other, all within a single trial. For two treatments, you’d typically have four arms: neither treatment, each treatment alone, and both treatments together. This approach is efficient and directly informs whether the combination provides extra benefit (or any negative interaction) beyond the individual effects. A parallel-group design, by contrast, assigns one treatment per group without systematically testing the combination of factors in the same study. A crossover design has participants receive multiple treatments in sequence, focusing on within-subject comparisons and carrying over effects. Multicenter trials describe study sites rather than a design feature.