A population-based cervical cancer screening programme reports a reduction in cervical cancer mortality 5 years after introduction of VIA (Visual Inspection with Acetic acid) screening. A critic argues that this improvement may be due to 'length-biased sampling' rather than true benefit of the programme. Length-biased sampling means:

• A The screening test detects early disease before symptoms appear, giving the false impression of longer survival
• B Screening preferentially detects slow-growing, less aggressive tumors with longer sojourn times, making it appear more effective ✓
• C Patients who survive longer are more likely to be included in follow-up studies
• D Lead time added to survival time creates an artificial improvement in 5-year survival rates

Explanation

Length-biased sampling occurs because screening detects a disproportionate number of slow-progressing, less lethal tumors (which spend more time in the preclinical detectable phase — longer 'sojourn time') compared to rapidly progressing, aggressive tumors that are less likely to be detected in a periodic screen. This creates an apparent survival benefit for screened cancers that may not translate to true population-wide mortality reduction. It is distinct from lead time bias (option D), where added lead time falsely inflates 5-year survival statistics without extending actual life. Both biases threaten the validity of survival comparisons between screened and unscreened populations.

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

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Written and medically reviewed by the StethoPrep medical team.