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Slowly progressing tumors have more opportunity than faster ones to
be detected by screening. In addition, slowly progressive tumors
take longer to lead to death than faster ones. Therefore, the
screen-detected cancers will appear to have an increased survival
after diagnosis, giving the mistaken impression that screening
leads to improved survival. In reality, the improved survival
is a result of these cancers being more slowly progressing.
Thus, the survival rate of a group of people with screen-detected
cancers will be artificially increased due to length time bias
compared with the survival rate of those with non screen-detected
cancers.
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