Which of the following is the most important criterion for assessing the internal validity of a randomized controlled trial?

Explanation

## Internal Validity in RCTs **Key Point:** Internal validity refers to the extent to which the study design and conduct allow valid causal inference about the effect of the intervention — it answers: "Did the intervention cause the observed outcome in THIS study?" ### Determinants of Internal Validity **High-Yield:** The cornerstone of internal validity is **adequate randomization** because it: 1. Minimizes selection bias 2. Distributes confounding variables equally between groups 3. Ensures baseline comparability (in expectation) 4. Allows unbiased estimation of treatment effect ### Internal vs. External Validity | Dimension | Definition | Key Criterion | Example | |-----------|-----------|---------------|----------| | **Internal Validity** | Can we trust the causal inference in THIS study? | Randomization, baseline balance, low attrition | Did the drug truly cause the benefit observed? | | **External Validity** | Can findings apply to other populations? | Generalizability, representativeness | Do results apply to my patient population? | **Mnemonic:** **BIAS** — factors threatening internal validity: - **B**aseline imbalance (confounding) - **I**nadequate randomization (selection bias) - **A**ttrition and loss to follow-up - **S**election into subgroups post-randomization ### Why Other Options Are Wrong **Warning:** Do not confuse internal validity with: - **Sample size/power** — these affect precision and Type II error, not causal inference validity - **Generalizability** — this is external validity, a separate concept - **Publication prestige** — journal impact does not validate study design **Clinical Pearl:** A small, well-randomized trial has better internal validity than a large, poorly-randomized observational study, even though the latter has more participants. [cite:Park 26e Ch 8]