In a randomized controlled trial comparing two antihypertensive drugs, randomization fails to balance a major confounder (age) between groups despite adequate sample size. Which of the following BEST describes this phenomenon?

• A Chance imbalance ✓
• B Selection bias
• C Confounding by indication
• D Attrition bias

Explanation

Randomization ensures that confounders are distributed between groups by chance, but in any individual trial — especially with small sample sizes — chance imbalance can still occur, leaving residual confounding. This is not selection bias (which relates to non-random allocation) nor confounding by indication (which is a problem in observational studies where the severity of disease influences treatment choice). Attrition bias arises from differential dropout after trial onset.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.