From Population Norms to Personal Baselines: A Paradigm Shift in Functional Health Surveillance

Abstract Modern screening programmes compare individuals to population averages—a practice inherited from the statistical conventions of 20th-century epidemiology. Yet biological variation within a person over time is typically two to four times smaller than variation between individuals. 1 A clinically dangerous change in one person may therefore be entirely invisible when assessed against population reference ranges. We present a decade of prospective evidence from 560 individuals demonstrating that monitoring personal gait-signature deviation—the Personal Deviation Index (PDI)—dramatically outperforms population-norm comparison for predicting cerebrovascular disease (AUC 0.87 vs 0.68; p < 0.001). We argue that this finding reflects a general biological principle with implications far beyond gait analysis: the future of screening lies in comparing each person to themselves, not to everyone else.