Length bias Legend Screening Disease onset Slowly progressive disease Rapidly progressive disease death Disease onset Legend Length bias Screening identifies 2 cases of rapidly progressive disease and 5 cases of slowly progressive disease Length bias also provides a mistaken view of the success of a screening program. Imagine some cases have a brief illness and others have a long course. Implementing a screenign program will identify more of the long-course cases than the short. If you calculate survival based on these, it will appear longer on average than is really the case. Note: The incidence of rapidly progressive disease is equal to that of slowly progressive disease Screening 1