An NHS screening programme finds that people diagnosed through screening live longer after diagnosis than those diagnosed with symptoms, but overall death rates from the disease are unchanged. Which bias best explains this?
Answer and explanation
Correct answer: D. Lead-time bias
- Step 1: Screening brings forward the time of diagnosis without altering the time of death.
- Step 2: Survival from diagnosis appears longer, but the disease has not been delayed. Mortality, not survival time, is the correct outcome.
Answer
Lead-time bias.
Why not the others: Selection bias from loss to follow up: Loss to follow up does not explain a longer apparent survival. Recall bias: Recall bias concerns how past exposures are remembered. Observer bias from unblinded outcome assessment: Observer bias concerns measurement, not timing of diagnosis.
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