In this chapter we give a brief overview of observational studies, also referred to as epidemiological studies (as mentioned above, we use these terms interchangeably). The rest of the book is dedicated to clinical trials, which belong to the category of experimental studies (see chapter 2). We decided to devote some space to observational studies for three reasons: first, data from epidemiological studies are often required to plan and interpret clinical trials; second, to better understand the basic principles of experimental studies it is useful to understand those of observational studies; third, some of the methods of data analysis are common to both types of study. We will present a comparison between observational and experimental studies later in the book, once we have discussed experimental studies in detail (chapter 9). The reader who has a specific interest in epidemiology will find only general concepts in this chapter. For more on this topic we recommend the textbooks by Hennekens and Buring , Lilienfeld and Lilienfeld  and Miettinen  among many others.
KeywordsLung Cancer Confounding Factor Transient Ischemic Attack High Density Lipoprotein Biological Plausibility
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