A researcher conducts a cohort study to examine the association between smoking and lung cancer. At baseline, smokers who already have undiagnosed early lung cancer are more likely to quit smoking before enrolment, making them appear as non-smokers. This introduces which specific bias?

• A Neyman (prevalence-incidence) bias
• B Protopathic bias ✓
• C Berksonian bias
• D Length-biased sampling

Explanation

Protopathic bias (also called reverse causation bias) occurs when a preclinical phase of the disease itself causes exposure status to change before diagnosis, making the exposure appear protective or unrelated. Here, incipient lung cancer causes smoking cessation, so those cases appear as non-smokers at enrolment. Neyman bias refers to exclusion of fatal early cases from prevalence studies. Berkson's bias arises from differential hospital admission rates. Length-biased sampling relates to screening studies favouring slower-progressing disease.

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

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Written and medically reviewed by the StethoPrep medical team.