A case-control study investigates the association between dietary fat intake and colorectal cancer. Participants are asked to recall their diet from 5 years ago. The OR comes out as 2.8, but a subsequent cohort study yields an RR of 1.4. The most likely explanation for the discrepancy is:

• A Lead-time bias affecting the cohort study
• B Healthy worker effect operating in the case-control study
• C Neyman (prevalence-incidence) bias in the cohort study
• D Differential recall bias inflating the OR in the case-control study ✓

Explanation

Cases with colorectal cancer are more motivated to recall past exposures accurately or over-report harmful exposures than healthy controls, producing differential recall bias that inflates the OR. Lead-time bias applies to screening studies. Healthy worker effect is an occupational cohort phenomenon. Neyman bias (selective survival) affects prevalent case studies but not incidence-based cohort studies.

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

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