Length-time bias in cancer screening refers to:
- A Screening preferentially detects slowly progressive tumours with long sojourn time, overrepresenting indolent disease ✓
- B Participants in screening programmes tend to be healthier than the general population
- C Earlier diagnosis increases apparent survival time without altering natural history (lead-time bias)
- D Tumours detected by screening are diagnosed at a younger age, biasing age-standardised survival
Correct answer: A. Screening preferentially detects slowly progressive tumours with long sojourn time, overrepresenting indolent disease
Explanation
Length-time bias (length-biased sampling) occurs because screening is more likely to detect slowly progressive cancers with a long preclinical detectable phase (sojourn time) than fast-growing aggressive tumours, which present symptomatically between screening rounds ('interval cancers'). This makes screen-detected cancers appear to have better prognosis, falsely inflating survival statistics for screening. Lead-time bias, a distinct concept, refers to earlier detection advancing diagnosis timing without changing actual survival.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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