Epidemiologic approaches to the study of diseases of complex etiology

In this preparation I have discussed some of the concepts and methods of epidemiologic studies of diseases of complex etiology. I have pointed out some of the relationships between epidemiology and genetics and examined briefly the relatively new field of genetic epidemiology. Epidemiology is a comparative science and is based on asking questions about the who, where, and when of disease occurrence in an attempt to understand "why." As a comparative science, it is necessary to have measures that can be meaningfully compared between places and times. In epidemiology, the measures we use are rates, and we have examined some of the commonly used types and kinds of rates and the information that is necessary for their generation. To a large degree, epidemiology is concerned with determining if there is an association between a characteristic or exposure and a disease. Epidemiologic reasoning involves an iterative process of hypothesis generation and testing. I described some of the study designs that are used in attempting to identify etiologic factors in diseases of complex etiology. I noted that we usually begin with descriptive studies and then move on to analytic studies where associations between exposure and disease are examined. In discussing study designs, I pointed out that studies can be based on examination of associations at either the group or individual level. I discussed the basic aspects of four epidemiologic study designs: ecologic studies, cross-sectional studies, case-control studies, and cohort studies. Illustrative examples of the study designs were presented from the literature. In addition, some of the basic statistical approaches were noted that are used to determine if the association between exposure and outcome is greater than that expected to occur by chance alone. Some of the strengths and weaknesses of various study designs were commented on briefly.