Length time bias

{{short description|Type of bias in epidemiology}}

{{Distinguish|Lead time bias}}

File:Length time bias.svg

Length time bias (or length bias) is an overestimation of survival duration due to the relative excess of cases detected that are asymptomatically slowly progressing, while fast progressing cases are detected after giving symptoms. {{cn|date=May 2023}}

Length time bias is often discussed in the context of the benefits of cancer screening, and it can lead to the perception that screening leads to better outcomes when in reality it has no effect. Fast-growing tumors generally have a shorter asymptomatic phase than slower-growing tumors. Thus, there is a shorter period of time during which the cancer is present in the body (and so might be detected by screening) but not yet large enough to cause symptoms, that would cause the patient to seek medical care and be diagnosed without screening.{{cn|date=May 2023}}

As a result, if the same number of slow-growing and fast-growing tumors appear in a year, the screening test detects more slow-growers than fast-growers. If the slow growing tumors are less likely to be fatal than the fast growers, the people whose cancer is detected by screening do better, on average, than the people whose tumors are detected from symptoms (or at autopsy) even if there is no real benefit to catching the cancer earlier. That can give the impression that detecting cancers by screening causes cancers to be less dangerous even if less dangerous cancers are simply more likely to be detected by screening.{{Cite web | url=http://www.fpnotebook.com/Prevent/Epi/LngthBs.htm |title = Length Bias|website=FPnotebook.com|last=Moses|first=Scott|date=2014}} Cites:

• {{Cite journal |pmid = 11272300|year = 2001|last1 = Gates|first1 = T. J.|title = Screening for cancer: Evaluating the evidence|journal = American Family Physician|volume = 63|issue = 3|pages = 513–22}}
• {{Cite journal | doi=10.1016/s0025-7125(05)70423-5 |pmid = 8569290|title = Principles of Cancer Screening|journal = Medical Clinics of North America|volume = 80|issue = 1|pages = 1–14|year = 1996|last1 = MacLean|first1 = Charles D.}}
• {{Cite journal | doi=10.1016/s0025-7125(05)70169-3 |pmid = 10584597|title = Principles of Screening|journal = Medical Clinics of North America|volume = 83|issue = 6|pages = 1323–1337|year = 1999|last1 = Nielsen|first1 = Craig|last2 = Lang|first2 = Richard S.}}

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