A difficult bias to control can be attributed to the volunteers for screening who represent either a group with better health, lower mortality and are more likely to adhere to prescribed interventions; or a group of the "worried well" who are asymptomatic, at higher risk of mortality because of medical or family characteristics regardless of the screening. The second bias is the lead time which 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. Cases progressing rapidly will gain less lead time than those who progress slowly. The lead time can be controlled by calculating the age-specific mortality rates. The length of the preclinical phase may also affect the evaluation of a screening program as conditions with long preclinical phase are detected by screening at an earlier stage than those with short preclinical phase. 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.