Inclusion and exclusion criteria

I/E Criteria - The Study Gatekeepers

• Inclusion Criteria: Defines the target population. These are characteristics subjects must have to be eligible (e.g., specific disease, age range).
• Exclusion Criteria: Characteristics that disqualify eligible subjects. Aims to reduce confounding, harm, or non-compliance (e.g., comorbidities, contraindications, pregnancy).
• Primary Goal: Create a homogenous study group to enhance internal validity and isolate the variable's effect.

⭐ Exam Favorite: Overly restrictive criteria boost internal validity but may significantly limit external validity (generalizability), as the study sample no longer reflects the real-world patient population.

Inclusion Criteria - Defining the Cohort

• Definition: A set of predefined characteristics that prospective subjects must have to be considered for participation in a clinical trial.
• Purpose:
  • Defines the target population to whom the study results will be generalizable.
  • Creates a homogenous sample to reduce the effect of confounding variables.
  • Ensures participants are appropriate for the research question and intervention.
• Common Criteria:
  • Demographics: Age, sex, ethnicity.
  • Clinical Characteristics: Specific diagnosis, disease severity, prior treatments, laboratory values (e.g., eGFR > 60 mL/min).
  • Geographic Location: To ensure follow-up feasibility.

⭐ High-Yield: Narrow inclusion criteria increase a study's internal validity (by reducing confounders) but may decrease its external validity (generalizability) to a broader population.

Exclusion Criteria - Refining the Sample

Applied after inclusion criteria to remove individuals for whom the study may be unsafe, unethical, or impractical, ensuring a refined and appropriate sample.

• Key Rationales:
  • Safety & Ethics: Protects participants at high risk of harm (e.g., pregnancy, severe comorbidities) or those unable to give informed consent.
  • Methodological Purity: Excludes individuals whose characteristics could confound results (e.g., specific concurrent medications) or who are unlikely to adhere to the study protocol (e.g., planning to move, substance abuse).
  • Practicality: Removes participants who are likely to be lost to follow-up.

⭐ Overly restrictive exclusion criteria are a major threat to external validity (generalizability), as the study sample may no longer reflect the real-world patient population.

Study Validity - The Balancing Act

• Strict criteria (narrow):

  • Creates a homogeneous sample, boosting internal validity by minimizing confounders.
  • Reduces external validity (generalizability) to broader, real-world populations.

• Loose criteria (broad):

  • Creates a heterogeneous sample, boosting external validity.
  • Risks ↓ internal validity due to varied patient characteristics and potential confounders.

⭐ The choice of criteria is a critical trade-off. Highly controlled trials (high internal validity) may not reflect real-world effectiveness, a frequent point of critique on exams.

High‑Yield Points - ⚡ Biggest Takeaways

• Inclusion and exclusion criteria are essential for defining the study population and ensuring the validity of an RCT.
• Inclusion criteria dictate the characteristics subjects must have, while exclusion criteria list traits they must not have.
• Strict criteria enhance internal validity by creating a homogenous group but can limit external validity (generalizability).
• Exclusion is critical for patient safety, protecting vulnerable individuals from potential harm.
• Inconsistent application can introduce selection bias.