Detection 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.