Prevention Book

http://www.fpnotebook.com/

Length Bias

Aka: Length Bias, Length-Time Bias
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  1. See Also
    1. Screening Test
    2. Contingency Grid (includes Statistics Example)
    3. Test Sensitivity
    4. Test Specificity
    5. Negative Predictive Value
    6. Positive Predictive Value
    7. Likelihood Ratio
    8. Number Needed to Screen
    9. Leadtime Bias
    10. Selection Bias
  2. Definition
    1. Screening over-represents less aggressive disease
  3. Example
    1. Aggressive disease has a short asymptomatic period
      1. Screening is unlikely to detect these patients early
      2. Clinical diagnosis will identify by symptoms
      3. Over-represented in clinical diagnosis cohort
    2. Less aggressive disease has longer asymptomatic period
      1. Disease often detected by screening
      2. Over-represented in screening cohort
    3. Screening program falsely appears to improve survival
      1. Less aggressive disease common in screening group
      2. Screening cohort has a better prognosis by definition
  4. References
    1. Gates (2001) Am Fam Physician 63(3):513-22
    2. MacLean (1996) Med Clin North Am 80(1):1-14
    3. Nielsen (1999) Med Clin North Am 83(6):1323-37

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