Length-time bias is a type of selection bias. It is a statistical distortion of results that frequently results in errors that lead to improper conclusions about the data being evaluated. This type of error is typically the result of interval lengths that are computed by the selection of the time interval at random points in time and space. This can be illustrated by the efficacy of cancer screenings. While annual cancer screening is highly effective when dealing with slow-growing tumors and cancers, it has little value when applied to fast-growing tumors that might not have been noticeable at the time of screening but can grow rapidly and kill the patient before the next annual screening is due.