bias is a systematic error in method of ascertainment, diagnosis or verification of cases.
Workup bias occurs due to incompletely or incorrectly diagnosed cases. The lead time bias
represents the amount of time by which the diagnosis has been advanced as a result of
screening and can give a false impression of increased survival among screen-detected
cases. Length bias could show beneficial effect of screening when differences in mortality
resulted from detection of less rapidly fatal diseases through screening while more fatal
diseases were diagnosed after development of symptoms regardless of the screening.