II. Definition
- Screening over-represents less aggressive disease
III. Example
- Aggressive disease has a short asymptomatic period
- Screening is unlikely to detect these patients early
- Clinical diagnosis will identify by symptoms
- Over-represented in clinical diagnosis cohort
- Less aggressive disease has longer asymptomatic period
- Disease often detected by screening
- Over-represented in screening cohort
- Screening program falsely appears to improve survival
- Less aggressive disease common in screening group
- Screening cohort has a better prognosis by definition