## Most Common Source of Bias in RCTs **Key Point:** Selection bias is the most frequent and problematic source of bias in RCTs because it occurs at the point of participant recruitment and allocation, affecting the fundamental comparability of study groups. ### Why Selection Bias Is Most Common Selection bias arises when: - Participants are not truly randomly allocated to treatment groups - Systematic differences exist between those who consent and those who decline - Recruitment procedures inadvertently favor certain types of participants - Baseline characteristics differ significantly between groups despite randomization ### Comparison of Common RCT Biases | Bias Type | Timing | Preventability | Frequency in Practice | |-----------|--------|-----------------|----------------------| | **Selection bias** | Recruitment & allocation | High (proper randomization) | **Most common** | | Measurement bias | Data collection | High (blinding, standardization) | Moderate | | Attrition bias | Follow-up phase | Moderate (retention strategies) | Common | | Publication bias | Reporting phase | Low (registry requirements) | Moderate | ### Prevention of Selection Bias 1. **Adequate randomization method** — computer-generated sequences, block randomization 2. **Concealment of allocation** — sealed envelopes, central randomization 3. **Prospective registration** — prevents selective reporting 4. **Intention-to-treat analysis** — preserves randomization benefit **High-Yield:** Selection bias is the reason why proper randomization and allocation concealment are the **hallmark features** that distinguish RCTs from other study designs. Without these, an RCT loses its primary advantage. **Clinical Pearl:** Even well-designed RCTs can suffer from selection bias if recruitment is done by clinicians with knowledge of treatment assignment (open-label designs), leading to preferential enrollment of certain patient types.
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