A screening programme for colon cancer using colonoscopy appears to reduce cancer mortality by 35% compared to no screening in a follow-up study. However, cancers detected by screening had been present for longer before diagnosis than clinically detected cancers. This makes the screened group's survival look better even without any true benefit—this is:
- A Lead-time bias
- B Volunteer bias
- C Length-biased sampling ✓
- D Overdiagnosis bias
Explanation
Length-biased sampling occurs because screening preferentially detects slow-growing tumours (which are present longer in the pre-clinical phase). Patients with slow-growing tumours have inherently better prognoses regardless of screening, making survival appear better in the screened group. This is distinct from lead-time bias, which artificially lengthens survival from the moment of earlier diagnosis even without changing the time of death. Overdiagnosis is the extreme form—detection of lesions that would never become symptomatic. Volunteer bias refers to health-seeking behaviour differences.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.