What is a pitfall of overly strict inclusion criteria?

• A. They Always Improve Generalizability
• B. Overly Strict Criteria Limit Generalizability
• C. They Have No Effect
• D. They Increase Sample Size

Answer: (B)

The key idea here is how inclusion criteria affect who the study results apply to. When criteria are overly strict, the study sample becomes very narrow and unrepresentative of the broader population you want to understand. That limits generalizability (external validity) because the findings may not hold true for everyone who would meet the real-world criteria or for patients seen in typical practice. For example, excluding people with common comorbidities or a wider age range can make results look different from what would occur in the real world, where such variations are common. While strict criteria can improve internal validity by reducing confounding, they trade off generalizability and often reduce the sample size because more potential participants are excluded. So the pitfall is that overly strict inclusion criteria limit how well the study’s conclusions apply beyond the study sample.