Which of the following best describes length bias in screening programmes?

• A Cases detected by screening survive longer because treatment begins earlier
• B Slowly progressing, better prognosis cases are preferentially detected by periodic screening ✓
• C Screening tests miss fast-growing tumours because they arise in the interval between screens
• D Volunteers for screening have healthier lifestyles than non-volunteers

Explanation

Periodic screening samples the prevalent pool repeatedly, and indolent lesions remain detectable for longer intervals, so slow-growing cases are overrepresented among screen-detected cancers. These cases have inherently better prognosis regardless of early treatment, inflating apparent screening benefit. Option A describes lead time bias, option C describes interval cancers which relate to test sensitivity, and option D describes volunteer or self-selection bias.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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