A case-control study of oral cancer recruits hospital controls from dentistry outpatient clinics. Because dentistry patients are more health conscious and less likely to use tobacco, this selection of controls introduces:

• A Misclassification bias
• B Neyman bias
• C Hawthorne effect
• D Berkson's bias ✓

Explanation

Berkson's bias (hospital admission rate bias) occurs when hospital controls are systematically different from the source population on the exposure of interest. Choosing dental clinic controls—who have lower tobacco use than the general population—biases the OR away from the null, making tobacco appear more strongly associated with oral cancer than it truly is. Misclassification bias affects exposure or disease categorisation. Neyman bias reflects survivor selection in prevalence studies.

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

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