In a randomized controlled trial of a new antidiabetic drug, patients in the intervention group received drug plus dietary counselling, while controls received placebo only. At end of study, HbA1c reduction was significantly greater in the treatment group. This trial suffers from:

• A Allocation bias
• B Attrition bias
• C Co-intervention bias (performance bias) ✓
• D Detection bias

Explanation

Co-intervention bias (a form of performance bias) occurs when participants in one arm receive additional interventions beyond the assigned treatment. Here, dietary counselling was given only to the drug group, making it impossible to attribute outcome differences solely to the drug. Allocation bias refers to non-random assignment. Attrition bias involves differential dropout. Detection bias arises from unblinded outcome assessment. To avoid co-intervention bias, both groups should receive identical co-treatments.

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

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