A cervical cancer screening programme preferentially detects slow-growing, indolent lesions because rapidly progressive cancers arise and become symptomatic in the interval between two screening rounds. If screen-detected cases therefore appear to have better prognosis than symptom-detected cases, the phenomenon is:
- A Length-time bias ✓
- B Neyman's bias
- C Berkson's bias
- D Hawthorne effect
Correct answer: A. Length-time bias
Explanation
Length-time bias arises because screening samples the population at one moment and is more likely to catch long-duration, slowly progressive disease, while aggressive short-duration cancers present between screens. Screen-detected cases are thus biologically milder and carry better prognosis independent of any true benefit. Neyman's bias is incidence-prevalence bias from missing fatal or recovered cases, Berkson's bias is admission-rate selection bias in hospital studies.
Reference: Gordis Epidemiology, 6th ed.
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