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Length bias occurs when screening preferentially picks up disease that
is progressing more slowly. Such disease has a tendency to have a better
prognosis. Thus those people whose disease is detected by screening
appear to do better. This is sometimes mistakenly considered to be a
benefit of the screening test. Length bias may occur when
carcinomas-in-situ are picked up with breast screening. These may be
slow-growing precursors to cancer. Their detection and treatment may
not noticeably change the incidence and mortality from the disease.
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