Length-biased sampling is a bias that affects which type of screening program, and in what direction does it distort survival estimates?

• A Cross-sectional prevalence surveys; it underestimates disease prevalence
• B Cohort studies of acute infections; it overestimates the case fatality rate
• C Case-control studies; it introduces recall bias into exposure assessment
• D Cancer screening programs; it makes screened cases appear to have longer survival than they truly do ✓

Explanation

Length-biased sampling arises in screening programs because slowly progressive (indolent) tumors have a longer detectable pre-clinical phase, so they are disproportionately detected by periodic screening compared to aggressive, fast-growing tumors. Screened patients therefore appear to have longer survival, not because screening altered the natural history, but because slower-growing cancers were preferentially selected. This is the basis of the 'lead time bias' and 'length bias' critique of screening study designs that compare screened versus unscreened cohorts without randomization.

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

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Written and medically reviewed by the StethoPrep medical team.