A metaphor for epidemiology: The sea of person time

Metaphors have had an important role to play in the theory and practice of epidemiology. Some well-known examples include "black boxes," the "web of causation," "shoe-leather epidemiology," the "ivory tower" and the ubiquitous "gold standard." Metaphors like these do not replace methods or principles but rather like memes can spark a creative response and thoughtful reflection. In this paper, I bring to the attention of epidemiologists a metaphor that originated forty years ago in a paper describing and explaining measures of disease incidence. The authors wrote about a "sea of population time" to represent how incident disease events-specifically, incidence density measures- occur in the two-dimensional space of person-time. A "sea of population time" or "sea of person time" seems ideally suited as a metaphor for creative and thoughtful development in epidemiology. The vast and varied characteristics of oceans provide a plethora of ideas that can potentially help us to think more deeply about the role and responsibilities of epidemiologists. As an example, consider the notion that epidemiologists' journey across this sea in their methodologically-laden and concept-heavy fishing boats searching for the causes of disease. At the same time, epidemiologists live in the sea itself subject to and thus at risk of all the same diseases that affect human populations. Storms on this imaginary sea could sink our boats causing us to rethink conceptual and methodological frameworks. Here I provide in lyric form examples that explore what might exist behind the sea of person time and on its shores.