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4.1 INTRODUCTION AND TERMINOLOGY
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4.1.1 Epidemiology: Definition and Scope
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Epidemiology is the study of distribution and determinants of disease and disease outcomes in human populations. The primary research question for epidemiologists is why individuals, or different populations, have different risks of disease or disease outcomes. Epidemiology is broadly focused, examining a full spectrum of disease determinants. These encompass biologic, environmental (including lifestyle), social, and economic factors. Consequently, concepts and methods from other disciplines, such as biological sciences and sociology, are critical to the design, conduct, and analysis of epidemiologic studies. Important contributions are also made from the field of statistics. Epidemiology provides a critical link between clinical or laboratory results and observed health effects in populations. An observational approach is often the only way to examine risk between disease and a specific risk factor because, for example, it is unethical to assign individuals to an arm of a randomized trial that exposes them to a suspected carcinogen.
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4.1.2 General Approach
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Epidemiology is often dichotomized into two disciplines: descriptive and analytic. Descriptive epidemiology primarily describes rates of disease in populations, either over time or across geographic areas or demographic subgroups. Analytic epidemiology focuses on individuals in a population, comparing diseased to nondiseased members to determine which factors increase risk for disease. Measures commonly used in descriptive epidemiology are described in Section 4.2. Measures used in analytic epidemiology are described in Section 4.3.
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4.1.3 Role of Epidemiology in Translational Medicine
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Whereas in vitro and in vivo studies using cell lines and animal models can control for a multitude of experimental conditions, this is difficult in human studies. Ethical and feasibility considerations prevent deliberate repeated exposures of known carcinogens to human subjects and randomization of human groups to receive either inferior therapies or environmental exposures; further, the genetic background of individuals cannot be manipulated ethically or logistically for experimental purposes even in this new era of organism-wide genome editing technologies such as CRISPR/Cas9 (Zhang et al, 2014). Although some experiments in humans may utilize intermediate subclinical end points that are reversible following brief exposure to a potential carcinogen, these intermediate end points are often not a replacement for the clinical outcome of interest. An example of this type of intermediate end point is the measurement of carcinogen-adduct formation in humans after a single exposure to a putative carcinogen. Such results may support the role of such a carcinogen, but do not provide evidence of increased rates of cancer in individuals exposed to the putative carcinogen. In general, the process of translating basic science discoveries into the clinical setting requires studies of humans, their biologic specimens, and associated clinical data. Such studies require consideration of many factors that can affect the development of disease or its outcome. This is particularly important in the omics era (eg, genomics, proteomics, epigenomics, metabolomics, transcriptomics, ...