Tuesday, May 31, 2011

Epidemiology

Epidemiology is literally defined as "the study of that which falls upon the common people" -- a definition which follows from the parsing out of the word into its Latin components:  epi (on or upon), demos (the common people), and logy (study).  The definition has evolved over the last century or so and in my first Introductory Epidemiology textbook, the following is offered:  "The study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems" (Aschengrau & Seage, 2003, Essentials of Epidemiology in Public Health).  In my intermediate textbook, the authors spare the reader an exposition of the term and instead summarize epidemiology as "the study of the distribution and determinants of health-related states or events in specified populations and the application of this study to control health problems" (Szklo & Nieto, 2007, Epidemiology:  Beyond the Basics).  And in my most advanced epidemiology textbook, a definition isn't even proffered until the third chapter:  "epidemiology [is] the study of the distribution of health-related states and events in populations" (Rothman, Greenland, & Lash, 2008, Modern Epidemiology).  Perhaps the most interesting characterization of epidemiology that I've come upon, however, is the one offered by Elizabeth Pisani, an HIV/AIDS epidemiologist and author of The Wisdom of Whores:  Bureaucrats, Brothels, and the Business of AIDS.  She summed up epidemiology as "sex and drugs" since, at least in her field of study, these are the two primary mechanisms by which HIV is transmitted.  Hers is technically not a definition -- more like a flippant answer to a question about what she does for a living -- but rather a view shaped by years of working in the gritty underworld of prostitution and drug injection. 

Epidemiology can further be further classified as descriptive or analytic.  In the former, the primary goal is to describe the epidemiologic trifecta:  person, place, and time.  The epidemiologist engaged in this activity wants to know which population a disease (health-related state) is affecting, where the disease is taking hold, and when it occurred.   Equipped with this information, an epidemiologist can identify high-risk populations and generate causal hypotheses, the testing of which falls under the purview of analytic epidemiology.  When articles in the popular press state that factor X is associated with premature death, factor Y contributes to higher virility, or factor Z leads to improved well-being among the newly married, an epidemiologist was testing a hypothesis about whether a risk factor is associated with a health outcome.  Note that epidemiology and the statistical techniques underlying the discipline have become so pervasive that a health outcome of interest may be beneficial and the risk factor(s) may not, in fact, be risky.  Modeling survival (longevity) among elderly persons -- rather than death -- and whether living longer is associated with "positive affect" is but one example of having both a positive health outcome and positive risk factor (the June 2011 issue of The American Journal of Epidemiology features an article exploring the aforementioned relationship).  Most epidemiologic research, including my own, is of the analytic variety. 

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