Adverse Selection & Survivor Bias

When a population's health metrics appear to improve, the improvement may stem from three distinct sources: (1) genuine progress in prevention or treatment that reduces disease incidence, (2) better survival of the diseased due to medical advances, which shifts the composition of the sick population, or (3) longer survival of the healthy due to successful prevention, which alters both the numerator and denominator of health statistics. Without stratified analysis, it is easy to misattribute the true cause of apparent progress.

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