Which bias occurs when screening tends to detect slower-progressing cases with better prognosis, biasing toward longer survival?

• A. Lead-time bias
• B. Selection bias
• C. Length-time bias
• D. Observer bias

Answer: (C)

Length-time bias arises in screening when the test is more likely to detect diseases that have a long preclinical, slowly progressing phase. These slower-progressing cases tend to have a better prognosis. So, the group detected by screening is enriched for less aggressive disease. As a result, survival after diagnosis appears longer in the screened individuals even if the actual time of death from the disease hasn’t changed. In other words, screening is catching the disease earlier but more often catching the cases that would progress slowly and live longer anyway, which biases survival statistics upward. This is different from lead-time bias, where earlier detection simply shifts the clock to the time of diagnosis without affecting when death occurs, and from other biases like selection or observer bias, which involve who is included or how outcomes are measured rather than the biology of disease progression.