What is the primary purpose of a parallel group design?

• A. To compare different treatments across arms
• B. To compare outcomes within the same subject
• C. To test dose-response relationships only
• D. To randomize across many centers

Answer: (A)

In a parallel group design, participants are randomized into separate groups, each receiving a different treatment, and these groups are followed concurrently. The main goal is to compare the outcomes between these treatment groups to determine which intervention works better or has a more favorable safety profile under similar conditions. This setup minimizes bias by ensuring that comparisons are made between groups at the same time, avoiding carryover effects that can occur when the same participant receives multiple treatments. It isn’t about comparing outcomes within the same person (that would be a within-subject or cross-over design), nor is it limited to exploring dose–response relationships, and it isn’t defined by multi-center randomization, even though a parallel trial can be conducted across multiple sites. Therefore, the primary purpose is to compare different treatments across arms.