How does precise specification of the source population and eligibility criteria affect internal validity?

• A. Clear criteria increase selection bias and reduce transportability
• B. They have no effect on internal validity
• C. Clear criteria reduce selection bias and improve transportability
• D. They only affect external validity

Answer: (C)

Defining the source population and eligibility criteria precisely helps control who is included in the study and from where they come, which is essential for internal validity. When these criteria are clear, the exposed and unexposed groups come from the same well-defined population, making their selection more comparable. This reduces selection bias because differences in outcomes are less likely to reflect who was chosen rather than the actual exposure effect. At the same time, clarity about who counts as part of the study clarifies to whom the results apply, improving transportability by showing exactly which populations the findings can generalize to.

If criteria are vague, you risk including participants who differ in ways related to the outcome, which can bias results and cloud the true association. Conversely, criteria don’t just affect external validity; they directly support internal validity by ensuring valid comparisons.