Length Bias


Length Bias, Length-Time Bias

  • Definition
  1. Screening over-represents less aggressive disease
  • 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