1. 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