A randomised controlled trial assigns eligible hypertensive patients to intervention or control by a random number table. Despite randomization, baseline characteristics reveal that the intervention group had significantly more diabetics. This is best explained by:

• A Selection bias in enrollment
• B Measurement bias in outcome assessment
• C Random chance imbalance — expected in small trials ✓
• D Allocation concealment failure

Explanation

Randomization does not guarantee perfect balance of all characteristics — it ensures that any imbalance is due to chance rather than systematic bias. In small RCTs, chance imbalances in baseline characteristics can occur and should be reported and accounted for in analysis (e.g., by stratified analysis or covariate adjustment). Allocation concealment failure could lead to selection bias during enrollment, not post-randomization imbalance. Measurement bias affects outcome assessment, not baseline characteristics.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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