In a randomized controlled trial of a new anti-hypertensive, participants in the treatment arm also spontaneously reduce salt intake because they know they are being treated. The observed blood pressure reduction in the treatment arm is thus larger than the drug effect alone. This phenomenon is BEST described as:

• A Hawthorne effect
• B Co-intervention bias ✓
• C Performance bias
• D Attrition bias

Explanation

Co-intervention bias (also called contamination in reverse) occurs when participants in one arm receive additional, unplanned interventions (here, dietary change) that influence the outcome, making the treatment effect appear larger or smaller than it truly is. The Hawthorne effect is behaviour change due to being observed regardless of treatment. Performance bias refers to differences in care provided to each group by investigators. Attrition bias arises from differential dropout.

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.