A researcher designs a study where participants with and without a risk factor are observed over time. Midway through, a new treatment becomes available that prevents the disease and is preferentially given to the high-risk group. This introduces which type of bias?
- A Attrition bias
- B Confounding by indication ✓
- C Protopathic bias
- D Lead time bias
Correct answer: B. Confounding by indication
Explanation
Confounding by indication (also called channeling bias) occurs when the decision to give a treatment or intervention is based on the risk status of the participant, making the treatment itself a confounder. Here, the new treatment given preferentially to high-risk participants will make the high-risk group appear to have lower disease rates than expected, confounding the risk factor-disease relationship. Attrition bias involves differential loss to follow-up; protopathic bias involves giving treatment based on early symptoms of the disease.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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