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Lead time is the period between early detection of disease and the time of its usual clinical presentation.
LEAD TIME BIAS
When evaluating the effectiveness of the early detection and treatment of a condition, the lead time must be subtracted from the overall survival time of screened patients to avoid lead time bias. Otherwise early detection merely increases the duration of the patients' awareness of their disease without reducing their mortality or morbidity. Numerous cancer screening procedures were thought to improve survival until lead time bias was addressed.