A screening programme detects breast cancer earlier than routine clinical diagnosis, yet the total lifespan of screened and unscreened women is identical. Survival measured from diagnosis appears longer in the screened group. This apparent benefit is best explained by:

• A Length bias
• B Hawthorne effect
• C Recall bias
• D Lead time bias ✓

Explanation

Lead time is the interval between detection by screening and the usual clinical diagnosis. Screening advances the date of diagnosis without delaying death, so survival from diagnosis looks inflated even though the natural history is unchanged. Length bias refers to over-representation of slowly progressing disease in screened populations, which is a different mechanism. Recall bias concerns exposure ascertainment and the Hawthorne effect concerns behaviour change under observation.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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