What is the relationship between trial design and participant selection?

• A. The scientific goal and purpose of the trial should be considered to avoid unnecessary exclusion of participant populations.
• B. Participant selection should be determined solely by site convenience.
• C. The trial's scientific goal and purpose guide participant selection to avoid unnecessary exclusion.
• D. Participant selection should maximize heterogeneity at all costs.

Answer: (C)

Trial design and participant selection are tightly connected: the scientific goal and intended use of the study results determine exactly who should be studied. Inclusion and exclusion criteria are set to enroll a group that can answer the research question safely and with interpretable data, avoiding unnecessary restrictions that would limit how broadly the results apply. For example, a pragmatic trial meant to reflect real-world use benefits from broader enrollment, while an explanatory trial focused on a specific mechanism or outcome might tighten criteria to reduce variability and better isolate the treatment effect. The aim is to minimize unnecessary exclusion because overly strict or unjustified criteria can bias results and limit applicability to the population that would use the intervention. Choosing participants solely for site convenience or pursuing maximal heterogeneity without regard to the scientific question would undermine the ability to draw valid, generalizable conclusions from the trial.