An investigator conducted formal evaluation for a screening program and found out that survival among screened individuals appears longer than that for non -screened individuals . What type of screening bias did she introduce? a . Length-biasec sampling b. Lead-time bias c. Recall bias d. Volunteer bias

The correct answer is b. Lead-time bias.<br /><br />Lead-time bias occurs when the detection of a disease is advanced, but the overall survival time remains unchanged. In this scenario, the investigator found that survival among screened individuals appears longer than that for non-screened individuals. This suggests that the screening program may have led to earlier detection of the disease, but it did not necessarily improve the overall survival time.<br /><br />Lead-time bias can occur when screening programs detect diseases at an earlier stage, but the treatment and prognosis remain the same. The apparent improvement in survival is due to the earlier detection, rather than any actual improvement in the disease outcome. This can lead to an overestimation of the effectiveness of the screening program.<br /><br />In contrast, length-bias sampling (option a) refers to the tendency for screening programs to detect only those individuals with less aggressive or slower-progressing forms of the disease, leading to an overestimation of the proportion of less severe cases. Recall bias (option c) occurs when there is a difference in the accuracy or completeness of recall between the screened and non-screened groups. Volunteer bias (option d) refers to the tendency for individuals who choose to participate in screening programs to be healthier or more motivated to maintain their health, which may not be representative of the general population.<br /><br />Therefore, the type of screening bias introduced in this scenario is lead-time bias.