A community-based screening programme for breast cancer reports improved 5-year survival compared to unscreened historical controls. A systematic reviewer cautions that this improvement may be entirely artefactual. Which TWO biases must be specifically addressed before attributing benefit to screening?

• A Lead-time bias and length-time bias ✓
• B Recall bias and observer bias
• C Hawthorne effect and placebo effect
• D Volunteer bias and Berkson's bias

Explanation

Lead-time bias: screening advances the date of diagnosis without necessarily changing the date of death; the apparent improvement in survival merely reflects the longer time of 'being a known patient.' Length-time bias: screening preferentially detects slow-growing tumours (which have a longer preclinical detectable phase), making screened cases appear to have better prognosis than unscreened cases even if the programme provides no mortality benefit. Both biases can give the false impression of improved survival. Only RCTs measuring cause-specific mortality (not survival) can determine true benefit.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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