Epidemiology and the Humanities
A Metaphor for Epidemiology: Return to the Sea of Person Time Douglas Weed* Douglas Weed
Metaphors are an essential if underappreciated part of epidemiology’s conceptual foundation and everyday practice. The web of causation, weight of evidence, and gold standard are familiar examples. Less familiar but no less relevant is the metaphor of the sea of person time. Introduced to epidemiology in 1982 in a paper explaining the notion of incidence density, the sea of person time (or population time as it first appeared) illustrated how disease events in 2-dimensional space and time formed the numerator of the incidence measure. The y-axis represented the population; the x-axis was time. As a metaphor, the sea of person time expands to its full 3-dimensional significance. We epidemiologists are travelers on this “sea” in our method-laden boats as we seek explanations for the disease events that we “catch” in our fishing expeditions. At the same time, we exist as denizens of the population—we live in this “sea”—and we are at risk for the same diseases. The sea of person time represents this fundamental duality of our practice and our human existence. In this paper, a traveler on the sea of person time seeks to cross it and discover what lies on its distant shore. The form is an epic lyric poem with hand-drawn illustrations. The traveler has many adventures along the way. He witnesses storms on the turbulent sea, a loss of navigation, the death of his guide, and an appreciation of his own mortality.