Abstract: Just as the occurrence of asthma and allergies can be studied at many different levels including populations, individuals, organs, tissues, or cells, the causes of asthma can be studied at these different levels. All of these approaches are potentially useful, and individual researchers will focus on different levels of analysis depending on their training, areas of interest, and availability of funding . In the past the major contribution of epidemiology to the study of chronic diseases has been on the population level, including analyses of patterns of disease prevalence and incidence across demographic, geographic, and oral factors (person, place, and time). In particular, many of the epidemiologic hypotheses concerning the causes of cancer and chronic diseases such as coronary disease have stemmed, at least in part, from geographic comparisons . It could be argued that the striking international differences in cancer incidence might not have become apparent if the cancer incidence analyses had been confined to countries with similar lifestyles, because the differences in cancer incidence (and the lifestyle-related risk factors that cause the incidence patterns) in many instances would not have been sufficiently great. Whole populations or regions of the world may be exposed to risk factors for disease (eg, high levels of cholesterol and low levels of antioxidants in the diet), and the associations of these factors with disease may become apparent only when comparisons are made between populations, or between regions of the world, rather than within populations .
Pediatric Clinics of North America 2003; 50(3): 539-53.