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Length BiasAka: Length-Time Bias

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  1. Definition
    1. Screening over-represents less aggressive disease
  2. 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
  3. References
    1. Gates (2001) Am Fam Physician 63(3):513
    2. MacLean (1996) Med Clin North Am 80(1):1
    3. Nielsen (1999) Med Clin North Am 83(6):1323

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