Chapter 1

### EPIDEMIOLOGIC CONCEPTS

Epidemiology is the study of the distributions and the etiologic factors of diseases in human populations. Epidemiologists employ a variety of designs and statistical procedures to identify and assess potential etiologic factors. Unless a particular exposure (procedure) is beneficial, most studies cannot ethically be experimental in design, such as randomized clinical trials. Therefore, epidemiologists are typically relegated to conducting observational studies of populations. As a contrast, in experimental studies, the investigator has much more “control” over the many exposures study subjects may encounter, whereas in observational studies, the investigator is relegated to being an “observer” of the exposure of subjects in a study population. In some instances, such exposures may not be known or cannot be adequately controlled for. Thus, observational epidemiology studies provide us with associations and do not “establish” causation.

The 2 primary types of observational studies that epidemiologists utilize are cohort or case-control studies. The cohort study approach, sometimes referred to as a prospective study design, starts with one group of individuals in a defined population exposed to a particular agent and compares the risk of disease/outcome in that group to a second group of individuals from the population not exposed to the same agent. These studies tend to involve large populations or long periods of follow-up time to establish the risk of disease/outcome reliably between the 2 groups. The second main type of study design—case-control studies (sometimes referred to as a retrospective study design)—is the reverse of the cohort approach. That is, the epidemiologist identifies a group of individuals from the population with the disease/outcome of interest and compares the frequency of the desired exposure/factor in that group to a second group of individuals from the population who do not have the disease/outcome. This design receives the moniker of retrospective because it starts with the disease and looks “backward” for factors that affect risk. Case-control studies, compared to cohort studies, tend to be more economical because one does not have to enroll large numbers of individuals, many of whom will never develop the disease/outcome that is of interest for study.

Associations in these studies are assessed by calculating “risk estimates.” The typical measure calculated in cohort studies to estimate how large an association is between a factor of interest and a disease/outcome of interest is known as the relative risk. A relative risk of 1.0 indicates that risk of disease in the group exposed to the risk factor is the same as the risk of disease in the group not exposed. Relative risks more than 1.0 indicate that risks are higher in the exposed group, whereas relative risks less than 1.0 indicate that risks are lower in the exposed group when compared to the unexposed. The typical measure used in case-control studies to estimate how large an association is between a factor of interest and a disease/outcome of interest is known as the odds ratio. Similar to the relative risk ...

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